In mammals, many aspects of physiology vary over the 24-hour day in a predictable, periodic manner. Hormones, such as thyroid-stimulating hormone and melatonin, as well as cardiovascular and autonomic measures such as heart rate, blood pressure, and body temperature all have a predictable diurnal pattern. It is important to understand that such variation is generated internally; that is, such variation represents a fundamental, endogenous circadian variation. Circadian variability is typically distinguished from the numerous exogenous factors that can influence the physiologic parameters in question by its periodic pattern (Turek & Zee, 1999). For example, sleep lowers body temperature, and physical activity, upright posture, food intake, and the state of wakefulness increase body temperature. Such exogenous masking factors can affect body temperature at any point in the 24-hour day. Thus, the behavioral act of going to sleep lowers body temperature, but the fall in body temperature during wakefulness also heralds the onset of sleep and incipient sleepiness. Nonetheless, when all such factors are strictly controlled (by having volunteers confined to supine bed rest with restricted activity, sleep deprivation, hourly isocaloric food intake, and controlled lighting conditions), body temperature continues to oscillate in a highly predictable way, affected only by age, sex, and, perhaps, mental status.