Our bodies have a built in alarm clock which tells us when to sleep and when to awake. This alarm clock is called the circadian rhythm. The circadian rhythm regulates not only sleep, but other processes such as body temperature and the secretion of certain hormones such as melatonin, cortisol, and growth hormone. The “biological clock” is located at the base of the brain, inside the hypothalamus, in an area called the suprachiasmatic nucleus (SCN).

In a healthy person, circadian rhythms follow a characteristic pattern over a 24-hour cycle (FIGURE 2). Melatonin, a hormone closely associated with sleep, is mainly secreted just after bedtime. Temperature tends to fall in the evening; it is this drop in body temperature that is partly responsible for the desire to sleep at that time. In the early morning, temperature begins to rise, signaling the body that it is time to awaken. Normally, this system is regulated by a number of external cues of which the most important is daylight. When the system is disrupted (by travel across time zones or by working nights) the body will have conflicting signals for sleep and wakefulness. For example, when a person travels to Europe from the eastern United States there is a six-hour time difference. If the person arrives in Europe in the morning, their body will still be on Eastern U.S. time. Therefore, their body will think it is still night time, and as such their body temperature will decrease, melatonin will be released and they will have a strong desire to sleep. Likewise, the opposite is true for night time such that their body will be six hours behind and still in waking mode. Hence, all of the physiological changes associated with sleep such as drop in body temperature and release of melatonin will not be present. These disruptions cause what is known as jet lag.

Figure 2
Melatonin concentrations in a normal man at baseline and after six nights of reversed light. From Brzezinski A, NEJM 1997;336:186-95.

Topic Editor: Carl Bazil, M.D.
Last Reviewed:10/01/05