Abbreviations adj. adjective adv. adverb Cf. confer (see also) n. noun pl. plural v. verb

Greek Letters  alpha  delta  phi  rho  tau  psi

A

Acclimation n. A process consisting of physiological or behavioral changes occurring within the lifetime of an organism that reduce the strain caused by experimentally-induced changes in particular climatic factors such as ambient temperature and photoperiod. // v. = acclimate // adj. = acclimated. // Cf. Acclimatization and Adaptation.

Acclimatization n. A process consisting of physiological or behavioral changes occurring within the lifetime of an organism that reduce the strain caused by the naturally-occurring changes in climatic factors associated with the seasons. // v. = acclimatize // adj. = acclimatized. // Cf. Acclimation and Adaptation.

Acrophase n. The time at which the peak of a rhythm occurs. Note: Originally, acrophase referred to the phase angle of the peak of a cosine wave fitted to the raw data of a rhythm (time series). When the term is applied to the actual rhythm, the acrophase will likely vary from cycle to cycle. Unit of measurement: hours (h) or degrees of circumference (°) in relation to an absolute or arbitrary reference. Caveat: The official units of time and of plane angle in the International System of Units are, respectively, the second and the radian. // Cf. Cosinor and Peak.

Action spectrum n. A function relating the effect of electromagnetic radiation to the wavelength of the radiation. // pl. = action spectra.

Actogram n. The graphical display of a time series along two time axes. The duration of a cycle (or predicted duration of a cycle) determines the length of each plot line. Successive cycles are plotted on successive lines. Note: On each line, values may be plotted in digital format (all-or-none data points) or analog format (using compressed Y axes). // Cf. Cartesian plot.

Adaptation n. 1. A decrease in the responsiveness of a sensory mechanism resulting from previous or continuing stimulation. 2. A process consisting of physiological or behavioral changes, occurring within the lifetime of an organism or as a result of genetic selection in a species, that reduce the strain caused by climatic or non-climatic changes in the environment. // v. = adapt // adj. = adaptive, adapted, adaptable.

Ad libitum adj. Freely available. // adv. = ad libitum. // Abbreviation: ad lib.

After-effects n. pl. Characteristics of a rhythm that result from previous stimulation, such as the longer free-running period observed after entrainment by a zeitgeber with period longer than that of the organism.

Alpha n. 1. The segment of a circadian cycle during which the organism is active. 2. The level of significance of a statistical test. // Cf. Rho.

Amplitude n. The difference between the peak (or trough) and the mean value of a wave. Note: For symmetrical waves, the amplitude is half the value of the range of oscillation. // Cf. Range of oscillation.

Anticipatory activity n. Activity exhibited prior to the initiation of a stimulus that is believed to be responsible for the activity. Note: In circadian physiology, the term anticipatory can be misleading. The organism's activity anticipates (precedes) the presentation of the synchronizing stimulus. However, there is no evidence that the anticipation is a volitional psychological state distinct from the phase angle of entrainment determined by the properties of the pacemaker and the zeitgeber.

Arrhythmia n. Absence of rhythmicity in a process that is normally rhythmic. // adj. = arrhythmic.

Astronomical time n. A standard of time based on astronomical observations. // Cf. Atomic time, Solar time, and Sidereal time.

Atomic time n. A standard of time based on the number of oscillations of the cesium-133 atom as its electrons move from high to low energy levels. Unit of measurement: second (s). // Cf. Astronomical time and Second.

B

Bathyphase n. The time at which the trough of a rhythm occurs. Note: This term is very rarely used. Unit of measurement: hours (h) or degrees of circumference (°) in relation to an absolute or arbitrary reference. Caveat: The official units of time and of plane angle in the International System of Units are, respectively, the second and the radian. // Cf. Acrophase.

Biorhythm n. A biological rhythm. Note: This term should be avoided because of its association with the unscientific theory of Fliess and Swoboda. // adj. = biorhythmic.

Bistability n. The property of showing entrainment to either the dawn component or the dusk component of a skeleton photoperiod. // adj. = bistable. // Cf. Skeleton photoperiod.

Bmal1 see Cycle

C

Cartesian plot n. The graphical representation of a time series on Cartesian coordinates (with time in the abscissa and the pertinent variable in the ordinate).

Chi-square () periodogram n. An implementation of the Enright periodogram that utilizes the chi-square distribution to determine the statistical significance of spectral power. // Cf. Enright periodogram.

Chronobiology n. The scientific study of biological rhythms. // adj. = chronobiological.

Chronome n. The full complex of rhythms and temporal trends in an organism. The chronome consists of a multi-frequency spectrum of rhythms, trends, and residual structures, including intermodulations within and among physiological variables as well as changes with maturation and aging. // adj. = chronomic.

Chronotherapy n. The treatment of disease based on principles of chronobiology. adj. = chronotherapeutic. // Cf. Chronobiology.

Circadian adj. Occurring or functioning in cycles of approximately 24 hours. Note 1: For most researchers of circadian rhythms, the definition of circadian must include the requirement of endogenous generation (as determined by the ability to freerun in constant conditions). Accordingly, the use of the term circadian in connection with nycthemeral rhythms whose endogenous nature has not been ascertained is acceptable only if there is a justifiable assumption of endogenesis. Note 2: Some researchers make the additional demand that a circadian rhythm be entrainable by a zeitgeber with a period in the circadian range (approximately 19 to 28 hours). // Cf. Nycthemeral and Daily.

Circadian hour n. The unit of time corresponding to 1/24 of the duration of a circadian cycle.

Circadian time n. A standard of time based on the free-running period of a rhythm (oscillation). Note: By convention, the onset of activity of diurnal organisms defines circadian time zero (CT 0). The onset of activity of nocturnal organisms defines circadian time twelve (CT 12). Unit of measurement: circadian hours (h) or degrees of circumference (°). Caveat: The official units of time and of plane angle in the International System of Units are, respectively, the second and the radian. // Cf. Circadian hour and Zeitgeber time.

Circalunar adj. Occurring or functioning in cycles of approximately one lunar cycle (29.5 days).

Circannual adj. Occurring or functioning in cycles of approximately one year. Note 1: For most researchers of biological rhythms, the definition of circannual must include the requirement of endogenous generation (as determined by the ability to freerun in constant conditions). Accordingly, the use of the term circannual in connection with annual rhythms whose endogenous nature has not been ascertained is acceptable only if there is a justifiable assumption of endogenesis. Note 2: Some researchers make the additional demand that a circannual rhythm be entrainable by a zeitgeber with a period in the circannual range (approximately 8 to 16 months).

Circatidal adj. Occurring or functioning in cycles of approximately one ocean tide (usually 12 hours and 25 minutes).

Civil time n. The standard system of time measurement based on solar time, with a mean solar day beginning at midnight. Unit of measurement: hours, calendar. // Cf. Solar time and Solar day.

Civil twilight n. The time of day between sunset (or sunrise) and the moment when the sun is 6° beneath the horizon. Other denominations: nautical twilight (12°) and astronomical twilight (18°).

Clk see Clock

Clock n. 1. A functional entity that indicates or records the time of day, usually by dividing the Earth's period of rotation into equal time intervals. 2. A gene that is an essential element in the molecular mechanism of circadian rhythmicity in animals. Abbreviation: Clk.

Cold-induced thermogenesis n. An increase in the rate of metabolic heat production, usually of endothermic animals, in response to cold exposure. Note: There are two forms of cold-induced thermogenesis: shivering (which results from increased contractile activity of skeletal muscles) and non-shivering thermogenesis (which results from other thermogenic mechanisms such as the activation of brown adipose tissue in mammals). Unit of measurement: watt (W).

Constant routine n. A research protocol intended to unmask the endogenous rhythmicity of physiological processes in human subjects. It usually involves constant bed rest under constant illumination with frequent, equally spaced meals.

Cosinor n. A procedure for the analysis of biological rhythms based on the fitting of a cosine wave to the raw data. Note: The single-cosinor procedure fits a single cosine function with a presumed period (anticipated on the basis of prior experience) to the time series and provides estimates of mesor, amplitude, and acrophase. More complex cosinor procedures utilize a fundamental function and one or more harmonics.

Crepuscular adj. Active mostly at dusk, at dawn, or at dusk and dawn. Note: Crepuscular is the adjective for the noun crepuscule. Matutinal crepuscule refers to dawn, and vespertinal crepuscule corresponds to dusk. An organism active only at dusk is said to be vespertinally crepuscular (although this expression is rarely used). // Cf. Dawn and Dusk.

Crest see Peak

Cry see Cryptochrome

Cryptochrome n. A gene (or family of genes) that is an essential element in the molecular mechanism of circadian rhythmicity in animals. In plants and invertebrates, the product of the Cryptochrome gene serves as a photic transducer in the entrainment pathway; in vertebrates, it is a core component of the clock mechanism. Abbreviation: Cry.

CT see Circadian time

Cyc see Cycle

Cycle n. 1. A single occurrence of a periodically repeated phenomenon. 2. A periodically repeated sequence of events. Note: Meaning 2 is synonymous with rhythm, although by tradition the reproductive rhythm is called the estrous cycle (or menstrual cycle), not the estrous rhythm. // adj. = cyclic. // Cf. Estrous and Rhythm. 3. A gene that is an essential element in the molecular mechanism of circadian rhythmicity in animals. The name Cycle is usually restricted to invertebrates. In vertebrates, the names Bmal1 and Mop3 are used. Abbreviation: Cyc.

D

Daily adj. Having the duration of a day (24 hours). Note: As defined here, daily is synonymous with nycthemeral. However, nycthemeral is a more precise term because daily may also mean "happening every day" or "happening once a day" in other contexts. // n. = day. // Cf. Circadian and Nycthemeral.

Dawn n. The time each morning at which daylight begins. // Cf. Dusk and Crepuscular.

Day n. 1. The 24-hour interval between two successive sunrises on Earth (solar day). 2. The interval of time between dawn and dusk. Note: The dual meaning of this term creates ambiguity in scientific language. However, tradition prevents the elimination of either meaning. Therefore, caution should be used to identify the intended meaning. When needed, terms such as "daylight hours" and "photophase" can be used to refer to meaning 2. // adj. = daily (meaning 1) or diurnal (meaning 2). // Cf. Lunar day, Siderial day, and Solar day.

Daylight-saving time n. Time during which clocks are set an hour or more ahead of standard time to provide more daylight at the end of the working day (from early April to late October in the Northern hemisphere). Note: In the United States, three-letter abbreviations are used to indicate daylight-saving time according to time zones; e.g., EDT (Eastern Daylight-saving Time) vs. EST (Eastern Standard Time). // Cf. Time zone.

DD n. Constant (continuous) darkness. // Cf. LD and LL.

Dead zone n. The section of a phase-response curve in which the stimulus evokes a null response. // Cf. Phase-response curve.

Delta phi n. Phase shift of a biological rhythm (oscillation). Cf. Phase shift.

Desynchronization n. Loss of synchrony between a rhythm and its zeitgeber (external desynchronization) or between two rhythms within an organism (internal desynchronization). // v. = desynchronize // adj. = desynchronized.

Diapause n. A period of inactivity in arthropods during which growth stops.

Diel see Nycthemeral

Diurnal adj. Occurring or active during the daytime. Note: In traditional English, diurnal may also mean daily. This second meaning should be avoided in circadian physiology to prevent ambiguity. // n. = daytime. // Cf. Daily and Nocturnal.

Double plot n. An actogram with two cycles per line. The second cycle on a line is the same as the first cycle on the following line. // v. = double plot. // adj. = double-plotted. // Cf. Actogram.

Dusk n. The time each evening at which daylight ends. // Cf. Dawn and Crepuscular.

E

Ectothermy n. The pattern of temperature regulation of organisms in which body temperature depends mainly on the behaviorally-controlled exchange of heat with the environment. // adj. = ectothermic. // n. = ectotherm. // Cf. Endothermy.

Electromagnetic spectrum n. The entire range of electromagnetic radiation that includes, in order of increasing wavelength, cosmic rays, gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.

Endogenous adj. Originating within an organism. // Cf. Exogenous.

Endothermy n. The pattern of temperature regulation of organisms in which body temperature depends on a high and controlled rate of metabolic heat production. Note: In physiology, endothermy refers to the production of heat within an organism; in chemistry, it refers to the uptake of heat from an outside source during a chemical reaction. // adj. = endothermic. // n. = endotherm. // Cf. Ectothermy.

Enright periodogram n. A mathematical procedure for the determination of periodicity in time series with equally-spaced data points. The Enright periodogram is based on the variances of different segments of the time series sequentially aligned by period. // Cf. Periodogram.

Entrain v. To synchronize a self-sustaining oscillation (or oscillator). Usage: The zeitgeber entrains the organism's clock (it "drags" the clock). In the passive voice, the organism is entrained by the zeitgeber. However, many authors have used the verb in a reverse transitivity: the organism entrains to the zeitgeber (meaning that the organism "enters the train" [or "succumbs to the dragger"]). // n. = entrainment // adj. = entrained. // Cf. Entrainment.

Entrainment n. The synchronization of a self-sustaining oscillation (such as a circadian rhythm) by a forcing oscillation (the zeitgeber). Under conditions of steady entrainment, the period of the self-sustaining oscillation conforms to that of the zeitgeber, and there is a stable phase relationship between the two of them. // v. = entrain. // Cf. Entrain.

Estivation n. A state of summer lethargy, with a reduction in body temperature and metabolism, in organisms that are temperature regulators when active. // v. = estivate. // adj. = estivating. // Cf. Hibernation.

Estrous adj. Relating to the reproductive cycle of non-primates. Usage: Estrus (n.) is one of the stages of the estrous cycle. // Cf. Menstrual.

Exogenous adj. Originating outside an organism. // Cf. Endogenous.

F

Food-entrainable oscillator n. A putative circadian pacemaker that can be entrained by a schedule of food restriction but not by a light-dark cycle.

Fourier analysis n. A mathematical procedure for the determination of periodicity in time series with equally-spaced data points. Fourier analysis is based on the decomposition of the time series into periodic components described by sine and cosine functions. // Cf. Periodogram.

Freerun n. The state of a self-sustaining oscillation (rhythm) in the absence of effective zeitgebers or other environmental agents that may affect the period of the oscillation. // v. = freerun. // adj. = free-running, freerunning.

Free-running period n. The period of an oscillation (rhythm) in the freerun state. // Cf. Freerun and Period.

Frequency n. 1. The number of times a specified phenomenon occurs within a specified time interval. Note: Frequency is the reciprocal of period (f = 1 / P). Unit of measurement: hertz (Hz). // Cf. Period. 2. A gene that is an essential element in the molecular mechanism of circadian rhythmicity in fungi. Abbreviation: Frq.

FRP see Free-running period

Frq see Frequency

H

Heterothermy n. The pattern of temperature regulation (of an endothermic species) in which the variation in body core temperature (either daily or seasonally) exceeds the species' limits of homeothermy. // adj. = heterothermic. // n. = heterotherm. // Cf. Endothermy and Homeothermy.

Hibernation n. A state of winter lethargy, with a reduction in body temperature and metabolism, in organisms that are homeothermic temperature regulators when active. // v. = hibernate. // adj. = hibernating. // Cf. Estivation.

Hibernaculum n. A place where one hibernates. // pl. = hibernacula. // Cf. Hibernation.

Homeostasis n. The regulatory process that ensures the relative constancy of the internal environment of an organism despite variations in the external environment. // adj. = homeostatic.

Homeothermy n. The pattern of temperature regulation (of an endothermic species) in which the variation in body core temperature (either daily or seasonally) is maintained within a narrow range despite much larger variations in ambient temperature. Note: There is not an exact definition of the limits of homeothermy. For any given species, the permissible range of variation must be at least as wide as the range of oscillation of the circadian rhythm of body temperature, which is as narrow as 0.5 °C in dogs and as wide as 6 °C in tree shrews. // adj. = homeothermic. // n. = homeotherm. // Cf. Poikilothermy.

Hypothermia n. The condition of an organism when body core temperature is below the normal range of oscillation. Note: Hypothermia may be regulated (e.g., during torpor and hibernation) or forced (when heat loss exceeds the capacity for heat production). // adj. = hypothermic.

I

Illuminance n. Density of the flow of energy, traveling in the form of electromagnetic waves as perceived by the human eye per unit time, incident on a surface. Unit of measurement: lux (lx) // Cf. Luminance.

Infradian adj. Occurring or functioning with a frequency lower than circadian (i.e., with a period longer than circadian). // Cf. Circadian and Ultradian.

Insomnia n. Chronic inability to sleep. Onset insomnia: inability to initiate sleep at the usual sleep time; termination insomnia: inability to maintain sleep until the end of the usual sleep interval. // adj. = insomniac.

Interval timer see Timer

Irradiance n. Density of the flow of energy, traveling in the form of electromagnetic waves per unit time, incident on a surface. Unit of measurement: W · m ^-2 // Cf. Radiance.

J

Jet lag n. A malaise associated with the disruption of bodily rhythms caused by high-speed air travel across time zones. // adj. = jet-lagged.

K

KaiABC n. A cluster of genes that is an essential element in the molecular mechanism of circadian rhythmicity in cyanobacteria.

L

LD n. A schedule of illumination consisting of a regular alternation of light and darkness each day. // Cf. LL and DD.

Light n. The visible segment of the electromagnetic spectrum (the interval of wavelengths from 400 to 700 nm). // Cf. Electromagnetic spectrum.

Light-dark cycle n. A schedule of illumination consisting of a regular alternation of light and darkness each day. // Cf. T cycle.

Light-entrainable oscillator n. A circadian pacemaker that can be entrained by a light-dark cycle.

Limit cycle n. The trajectory in phase space around which the values of state variables (oscillating components) change.

LL n. Constant (continuous) illumination. // Cf. LD and DD.

Lomb-Scargle periodogram n. A mathematical procedure derived from Fourier analysis and used for spectral analysis of time series with unequally-spaced data points. // Cf. Fourier analysis and Periodogram.

Luminance n. Luminous intensity per unit area leaving, passing through, or arriving at a surface in a given direction. Unit of measurement: cd · m ^-2. // Cf. Luminous intensity and Illuminace.

Luminous intensity n. Flow of energy traveling in the form of electromagnetic waves, as perceived by the human eye, per unit time per unit solid angle. Unit of measurement: candela (cd).

Luminosity n. A non-technical term referring to the intensity of illumination. Note: In optics, luminosity refers to the quality of giving light; in astrophysics, it refers to the absolute brightness of a celestial object. // v. = illuminate // adj. = illuminated // Cf. Luminance and Illuminance.

Lunar day n. 1. The 24.8-hour interval during which the Earth completes one rotation on its axis with respect to the Moon. 2. The 29.5-day interval between one sunrise and the next for an observer at a location on the Moon. // Cf. Siderial day and Solar day.

M

Masking n. Disruption in the expression of overt rhythms caused by an external agent without a direct effect on the period or phase of a pacemaker. // v. = mask // adj. = masked // Cf. Entrainment and Relative coordination.

Mean level n. A measure of central tendency of the distribution of instantaneous values of an oscillating variable (the average value around which the variable oscillates). Note: The mean level is often computed as the arithmetic mean of all measured values in one cycle. // Cf. Mesor.

Menstrual adj. Relating to the reproductive cycle of primates. // Cf. Estrous.

Mesor n. An estimate of central tendency of the distribution of values of an oscillating variable (the average value around which the variable oscillates). The mesor is a circadian rhythm-adjusted mean based on the parameters of a cosine function fitted to the raw data. Note: When a process is known to be rhythmic, and the data points are not equidistant or the sample size is small, the Mesor often provides a more appropriate unbiased estimator of central tendency than does the arithmetic mean of the raw data. // Cf. Mean level.

Metabolic rate n. Amount of metabolic activity per unit time. Note: In physiology, the amount of metabolic activity is usually determined as the amount of heat released by the metabolic process (or as the amount of oxygen consumed under aerobic conditions). Unit of measurement: watt (W).

Monochromatic adj. Pertaining to light of a single or narrow-band wavelength.

Mop3 see Bmal1

Morning-evening typology n. An arbitrary classification of organisms into morning types, evening types, and intermediary types according to their usual phase of entrainment under a light-dark cycle.

N

Nadir n. The lowest value of an oscillatory function. Note: This term is only occasionally used in circadian physiology as a synonym of trough (of a rhythm). // Cf. Trough.

Nocturnal adj. Occurring or active during the nighttime. // n. = night, nighttime. // Cf. Diurnal.

Non-photic adj. Of or relating to stimuli other than light. // Cf. Photic.

Nycthemeral adj. Having the duration of a day (24 hours). Note: The term diel has occasionally been used as a synonym of nycthemeral. // Alternative spelling: nyctohemeral. // Cf. Daily and Circadian.

O

Offset n. The end of alpha. // Cf. Alpha and Onset.

Onset n. The beginning of alpha. // Cf. Alpha and Offset.

Oscillator n. An entity capable of generating a periodic variation in the value of a physical or logical quantity, especially a regular variation above and below some mean value. // v. = oscillate // adj. = oscillatory // n. = oscillation. // Cf. Pacemaker.

P

Pacemaker n. A functional entity capable of generating endogenous rhythmicity and of imposing this rhythmicity on one or more other entities. Note: A pacemaker is an oscillator, but not all oscillators are pacemakers. // v. = oscillate, set the pace // adj. = pacemaking, oscillatory. // Cf. Oscillator.

Parametric adj. 1. Dependent on the parameters (particularly intensity and duration) of the zeitgeber. Note: In strict terms, all biological responses are parametric. However, the expression non-parametric entrainment is often used to denote entrainment that is primarily due to the timing (rather than the intensity or duration) of the synchronizing stimulus. Antonym: = non-parametric. 2. Dependent on the parameters (usually the mean and variance) of a distribution. Note: In inferential statistics, the most common parametric tests are the t test and the analysis of variance (ANOVA). Antonym: = non-parametric.

PAS domain n. A sequence of approximately 270 amino acids found in most of the central proteins involved in the molecular mechanism of the circadian clock in eukaryotes. (Per: period; ARNT: aryl hydrocarbon receptor nuclear transporter; Sim: single-minded protein)

Peak n. The point of culmination of an oscillatory function. Synonym: crest.

Per see Period

Period n. 1. The time elapsed for one complete oscillation or cycle (the distance in time between two consecutive peaks [or troughs, etc.] of a recurring wave). Note 1: Period is the reciprocal of frequency (P = 1 / f). Note 2: Period has a second, less precise meaning in English (period = an interval of time). To avoid ambiguity, this second meaning should be avoided in circadian physiology. Interval is usually an adequate substitute. Unit of measurement: hours (h), seconds (s), days (d), etc. // n. = periodicity. // Cf. Frequency. 2. A gene (or family of genes) that is an essential element in the molecular mechanism of circadian rhythmicity in animals. Abbreviation: Per.

Periodogram n. A function relating periodic components of a time series to their spectral power. Note: There are many types of periodograms (e.g., Fourier, Enright, Lomb-Scargle, etc.). If the type of periodogram is not specified, the periodogram is usually assumed to be a Fourier periodogram (i.e., a periodogram obtained by Fourier analysis). The term periodogram was first used in 1906 to describe a squared function of the sine and cosine terms of a Fourier transform.

Phase n. 1. The relative angular displacement between a periodic quantity and a reference angle (synonym: phase angle). 2. A distinct stage of a process (such as the luteal phase of the estrous cycle).

Phase angle n. The relative angular displacement between a periodic quantity and a reference angle.

Phase shift n. A discrete displacement of an oscillation along the time axis. Cf. Transients.

Phase-response curve n. A graphical description of how the magnitude of phase shifts induced by single stimuli depends on the phase at which the stimuli are presented.

Phi n. Phase of a biological rhythm (oscillation). // Cf. Phase.

Photic adj. Of or relating to light. // Cf. Non-photic.

Photoperiod n. The duration of the illuminated segment of a light-dark cycle. Usage: A "long photoperiod" refers to a light-dark cycle with a light segment longer than the dark segment. // Cf. Photophase and Scotophase.

Photoperiodism n. The property of being affected by changes in photoperiod. // adj. = photoperiodic. // Cf. Photoperiod.

Photophase n. The illuminated segment of a light-dark cycle. // Cf. Photoperiod and Scotophase.

Photoreceptor n. A structure or molecule that detects light. Note: In the mammalian retina, there are three classes of photoreceptors: rods, cones, and photosensitive ganglion cells.

Photorefractory adj. Temporarily (or permanently) nonresponsive to light. // n. = photorefractoriness.

Phototaxis n. Movement of an organism toward or away from a source of light. // adj. = phototactic. // Cf. Phototropism.

Phototherapy n. Treatment of disease by the use of light. // adj. = phototherapeutic.

Phototropism n. Plant movement or orientation in response to the location of a source of light. // adj. = phototropic. // Cf. Phototaxis.

Pineal gland n. A small endocrine gland lying between the superior colliculi (in the brain) and secreting the hormone melatonin.

Poikilothermy n. The pattern of temperature regulation (of an ectothermic species) in which body core temperature exhibits great variability as the result of variations in ambient temperature. Note: Many poikilothermic animals can minimize the influence of environmental temperature on body temperature by the use of behavioral responses. // adj. = poikilothermic. // n. = poikilotherm. // Cf. Homeothermy.

Power spectrum n. The distribution of variances associated with the periodic components of a time series.

Psi n. Phase angle of a biological rhythm (oscillation) in reference to the zeitgeber. // Cf. Phase angle.

Q

n. The ratio of the rate of a physiological process at a particular temperature to the rate at a temperature 10 °C lower. Note 1: For most physiological processes, Q₁₀ is approximately 2. Note 2: Q₁₀ values are of very little use if the logarithm of the rate is not an approximately linear function of temperature.

R

Radiance n. Radiant intensity per unit area leaving, passing through, or arriving at a surface in a given direction. Unit of measurement: W · sr ^-1 · m ^-2. // Cf. Radiant intensity and Irradiance.

Radiant intensity n. Flow of energy traveling in the form of electromagnetic waves per unit time per unit solid angle. Unit of measurement: W · sr ^-1.

Range of entrainment n. The range of periods of a zeitgeber capable of entraining a self-sustaining oscillation (rhythm). Note: Consistently with the transitivity of the verb entrain, the range of entrainment is a property of the zeitgeber, not of the organism. However, the same zeitgeber can have different ranges of entrainment for different organisms, which means that the range of entrainment is also a property of the organism (and, therefore, one can speak of the "range of entrainment of an organism"). // Cf. Entrain.

Range of oscillation n. The extent of variation of an oscillatory process (the difference between the maximum and minimum observed values). // Cf. Amplitude.

Relative coordination n. Partial entrainment of a rhythm (oscillatory process) by a zeitgeber, where the zeitgeber affects the period and phase of the pacemaker but not with enough strength to establish steady entrainment. // Cf. Entrainment and Masking.

Retino-hypothalamic tract n. The monosynaptic pathway that connects the retina (in the eye) to the hypothalamus (in the diencephalon).

Rho n. The segment of a circadian cycle during which the organism is inactive. // Cf. Alpha.

Rhythm n. A periodically repeated sequence of events. // adj. = rhythmic. // Cf. Cycle.

S

Second n. 1. The base unit of time of the International System of Units, defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. 2. A unit of angular measure equal to 1/60 of a minute (which is 1/60 of a degree). // Cf. Time.

Scotophase n. The dark segment of a light-dark cycle. // Cf. Photophase.

Self-sustaining oscillation n. An oscillation that can be sustained without external support. Antonym: damped oscillation. Usage: Pacemakers are capable of producing self-sustaining oscillations. Timers can produce only damped oscillations. // Cf. Pacemaker and Timer.

Set point n. A reference signal used by a homeostatic system to adjust the regulated variable. The difference between the set point and the current value of the regulated variable determines the magnitude of the error signal and, consequently, directs the corrective action to be taken. // Cf. Homeostasis.

Shift work n. A work schedule involving non-traditional working hours, usually during the evening and night.

Siderial day n. The 23.9-hour interval during which the Earth completes one rotation on its axis with respect to the stars. // Cf. Lunar day and Solar day.

Sidereal time n. A standard of time based on the apparent movement of the celestial sphere (i.e., the perceived sphere that contains the stars). Unit of measurement: hours, calendar. // Cf. Solar time.

Singularity n. The point of equilibrium in a limit cycle. // Cf. Limit cycle.

Skeleton photoperiod n. A light-dark cycle whose photophase consists only of brief photic stimulation at dawn, at dusk, or at dawn and dusk. // Cf. T cycle and Light-dark cycle.

Slave oscillator n. An oscillator that is driven or entrained by another oscillator. // Cf. Oscillator.

Solar day n. The 24.0-hour interval between one sunrise and the next for an observer at a location on Earth. Note: Because the solar day is not constant, solar day usually means mean solar day (the mean duration of a solar day). // Cf. Lunar day and Siderial day.

Solar time n. A standard of time based on the apparent movement of the sun in the sky (or the movement of a "fictitious" sun that moves uniformly along the celestial equator). Unit of measurement: hours, calendar. // Cf. Sidereal time.

Spectral analysis n. A mathematical procedure that evaluates the contribution of putative periodic components of a time series to the actual temporal profile of the series. // Cf. Fourier analysis.

Spectral power n. The relative magnitude of the variance associated with a period (or frequency) in the power spectrum of a time series. // Cf. Power spectrum.

Splitting n. The separation of a rhythm into two independent components. Note: Some researchers believe that the two components of a split circadian rhythm are controlled by distinct oscillators, often called the morning and evening oscillators. // v. = split // adj. = splitted.

Spring tide n. A tide of increased range that occurs about twice monthly at the new and full phases of the Moon.

State variable n. In a limit cycle, a variable that must be defined in order for the oscillatory state of the system to be known. Cf. Limit cycle.

Subjective day n. The segment of a circadian cycle during the freerun state that corresponds to the illuminated segment during entrainment by a light-dark cycle. Metaphorically, the organism or pacemaker "thinks" that subjective day is the daylight segment of a day. // Cf. Day and Subjective night.

Subjective night n. The segment of a circadian cycle during the freerun state that corresponds to the dark segment during entrainment by a light-dark cycle. Metaphorically, the organism or pacemaker "thinks" that subjective night is the night segment of a day. // Cf. Subjective day.

Suprachiasmatic nucleus n. A small group of nerve cells lying in the ventral hypothalamus and possessing the properties of a circadian pacemaker. // pl. = suprachiasmatic nuclei.

Synchronization n. 1. The action of causing two or more processes to proceed at the same rate. 2. The action of causing two or more processes to start at the same time (or to coincide in their phase of oscillation). Note: Meaning 1 is synonymous with entrainment. Meaning 2 is usually avoided by researchers of circadian rhythms. // v. = synchronize. // adj. = synchronized. // n. = synchronizer. // Cf. Entrainment.

Synchronizer see Zeitgeber

Synchrony n. Simultaneous occurrence. Usage: Generally, synchrony is a state, whereas synchronization is a process. // adj. = synchronous, synchronized // Cf. Synchronization.

T

T n. Period of a zeitgeber. // Cf. Tau.

T cycle n. A schedule of illumination consisting of a regular alternation of light and darkness with a period different from 24 hours. Note: T cycles often have a very short photophase, but a short photophase is not a required property. // Cf. Light-dark cycle and Skeleton photoperiod.

Tau n. Period of a biological rhythm (oscillation). // Cf. T.

Temperature compensation n. The property of preserving the rate of a biological process as the surrounding temperature changes. // adj. = temperature-compensated.

Thermogenesis n. Metabolic heat production. Note: There are four forms of thermogenesis in animals: obligatory thermogenesis (responsible for basal metabolic rate), diet-induced thermogenesis, cold-induced thermogenesis, and voluntary thermogenesis (resulting from exercise or external work). Unit of measurement: watt (W).

Thermotaxis n. Movement of an organism toward or away from a source of heat. // adj. = thermotactic.

Tim see Timeless

Time n. A nonspatial continuum in which events occur in apparently irreversible succession. Note: The current standard of time is atomic time. // adj. = temporal. // Cf. Atomic time.

Timeless n. A gene that is an essential element in the molecular mechanism of circadian rhythmicity in animals. Abbreviation: Tim.

Timer n. A device that measures the passage of time. // Cf. Clock.

Time series n. A set of observations of values in chronological order that is used to determine the effect of time on the values observed. // pl. = time series.

Time zone n. Any of the 24 longitudinal divisions of Earth's surface in which a standard time is kept. The standard time of each zone is the mean astronomical time of one of 24 meridians, 15° apart, extending east and west of the Greenwich meridian (in England). Note: Within the United States territory, there are eight official time zones: Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii-Aleutian, and Samoa.

Torpor n. A state of inactivity and reduced responsiveness to stimuli associated with a reduction in metabolism and body temperature. // adj. = torpid.

Transients n. pl. Temporary oscillatory states between two steady states. Note: Transients are often observed after phase shifts of circadian rhythms.

Trough n. The lowest value of an oscillatory function.

Twilight see Civil twilight and Crepuscular

Type 0 resetting n. A strong resetting of a pacemaker in which, on average, the new phase is the same regardless of when in the cycle the resetting stimulus was applied. Cf. Type 1 resetting.

Type 1 resetting n. A weak resetting of a pacemaker in which, on average, the new phase is a function primarily of the phase at which the resetting stimulus was applied. Cf. Type 0 resetting.

U

Ultradian adj. Occurring or functioning with a frequency higher than circadian (i.e., with a period shorter than circadian). // Cf. Circadian and Infradian.

W

Wave n. 1. A uniformly advancing disturbance in a medium. 2. The graphical representation of the prototypical cycle of a rhythm. // Cf. Cycle and Rhythm.

Wave form n. The shape of a wave (such as square, sinusoid, etc.). // Cf. Wave.

Wave length n. The distance between two successive peaks (or troughs, etc.) of a regular wave. Alternative spelling: wavelength. Unit of measurement: meter (m). // Cf. Period.

Z

Zeitgeber n. A synchronizing agent (a stimulus capable of resetting a pacemaker or synchronizing a self-sustaining oscillation). Note 1: As a German noun, Zeitgeber should always be capitalized. However, as an English noun of German origin it need not be capitalized. Note 2: The zeitgeber "gives" the local time, not the ability to keep time (which the organism already possesses). // v. = entrain, synchronize // adj. = entrained, synchronized.

Zeitgeber hour n. The unit of time corresponding to 1/24 of the period of a zeitgeber.

Zeitgeber time n. A standard of time based on the period of a zeitgeber. Note: Under standard 12:12 light-dark cycles, the time of lights on usually defines zeitgeber time zero (ZT 0) and the time of lights off defines zeitgeber time twelve (ZT 12). Unit of measurement: zeitgeber hours (h) or degrees of circumference (°). Caveat: The official units of time and of plane angle in the International System of Units are, respectively, the second and the radian. // Cf. Circadian time.

Zenith n. The point of culmination of an oscillatory function. Note: This term is only occasionally used in circadian physiology as a synonym of peak (of a rhythm). // Cf. Peak.

ZT see Zeitgeber time