These physiologic variables rise and fall with a period about 24 hours. Melatonin, for example, increases in the evening, plateaus during the night, and decreases in the morning. Cortisol rapidly increases in the early morning and gradually decreases during the day. Although external factors may influence the levels of these variables, these rhythms are endogenous. For example, although core body temperature may be influenced by external variables such as exercise and sleep, the temperature has a clear circadian rhythm even in subjects who are at bed rest and sleep deprived for over 24 hours.
Circadian rhythms have a marked impact on sleep.
Shifts in the timing of circadian rhythms can cause initial insomnia, early morning awakening, or excessive sleep. Circadian rhythms are usually coupled and have a consistent phase relationship. For example, temperature and sleep are coupled, with sleep occurring during the low temperature trough. However, under some circumstances these rhythms may be become uncoupled. In such studies, most subjects have a temperature rhythm period of greater than 24 hours. .