Circadian Rhythms in Man

Like many animals, humans have adjusted to the natural environment so as to be prepared for efficient activity during daytime and to rest at night. As the basis of this program, almost all structures and functions undergo regular changes within 24 hours. Such rhythms have been described in physiological parameters, such as hormone levels in blood plasma, blood pressure and heart rate, body temperature, and sleep propensity, and in behavioral parameters, such as mood, reaction time, computation speed, and performance in learning tasks. These rhythms normally keep distinct phase-relationships with each other, implying a high degree of internal temporal order. In principle, this rhythmicity is maintained when subjects are sleep deprived, when they are starved, or when they receive equal amounts of food at short intervals over the day. The timing of single meals, however, may influence the pattern of some variables, and the sleep-wake cycle can have phase-setting as well as masking effects on the instantaneous value of rhythms as especially known for body temperature and some hormones.