Practically every function in the human body has been shown to exhibit circadian rhythmicity. Under controlled conditions in the laboratory, the endogenous nature of the rhythmicity can be demonstrated. In ambulatory conditions, environmental factors and physical exertion can obscure or enhance the expressed rhythms. The three most commonly monitored vital signs are blood pressure (systolic and diastolic), heart rate, and body tem- perature. Because these variables exhibit daily rhythmicity, their normal values vary with the time of day. The table on the right shows the normative ranges for these variables.

For more information about circadian rhythms, visit the page on Circadian Rhythms.

Of the three major vital signs, blood pressure is the most irregular one under ambulatory conditions. In order to obtain the figure shown on the right, data from seven consecutive days had to be averaged. (The patient was a healthy sedentary adult male who used to sleep from 01:00 to 09:00 each day.)

Although the data evince considerable random variability, a daily pattern can be observed. Blood pressure falls during sleep (indicated by the grey rectangle), rises at wake-up time, and remains relatively high for the next 6 hours or so.

This patient had a mean systolic pressure of 117 mm Hg (diastolic pressure is not shown). Notice that values as low as 103 mm Hg are normal during sleep and that values as high as 133 mm Hg are normal in the first part of the day. Clearly, the concept of hypertension is dependent on the time of day, and a single measurement taken at the doctor's office can be very misleading (although, of course, systolic pressures above 140 mm Hg would characterize hypertension at any time of the day under sedentary conditions).

If physical exertion is avoided, the daily rhythm of heart rate is robust even under ambulatory conditions. As a matter of fact, ambulatory conditions enhance the rhythmicity because of the absence of physical activity during sleep time and the presence of activity during the wakefulness hours.

The figure on the right shows the values of heart rate of an adult sedentary male averaged over seven consecutive days. Heart rate is clearly lower during the sleep hours than during the awake hours. The mean heart rate for this patient was 69 bpm, with lows at 55 bpm and highs at 83 bpm.

Of the three major vital signs, body temperature is the one that has the most robust rhythm. The rhythm can be disrupted by physical exertion, but it is very reproducible in sedentary people. The figure on the right shows the values of oral temperature of an adult sedentary male averaged over seven consecutive days. Notice that the curve is much smoother than those of blood pressure and heart rate. Notice also that the concept of fever is dependent on the time of day. This patient had a mean tem- perature of 36.9°C, but tempe- ratures varied from 36.2°C to 37.4°C. At 05:00, a reading as low as 36.8°C would be indicative of (mild) fever. At 19:00, a rea- ding as high as 37.5°C would not be indicative of fever.