What makes us sleep?

To understand the importance of sleep, it is helpful to know something about the basic mechanisms of the sleep-wake cycle.

This 24-hour cycle is controlled by a combination of two internal influences: sleep homeostasis and circadian rhythms.

Homeostasis is the process by which the body maintains a “steady state” of internal conditions such as blood pressure, body temperature, and acid-base balance. The amount of sleep each night is also under homeostatic control. From the time that we wake up, the homeostatic drive for sleep accumulates, reaching its maximum in the late evening when most individuals fall asleep. Although the neurotransmitters of this sleep homeostatic process are not fully understood, there is evidence to indicate that one may be the sleep inducing chemical, adenosine. As long as we are awake, blood levels of adenosine rise continuously, resulting in a growing need for sleep that becomes more and more difficult to resist. Conversely, during sleep, levels of adenosine decrease, thereby reducing the need for sleep. Certain drugs, like caffeine, work by blocking the adenosine receptor, disrupting this process. Sleep loss results in the accumulation of a sleep debt that must eventually be repaid. When we stay up all night, for example, our bodies will demand that we make up each hour of lost sleep, by napping or sleeping longer in later cycles, or suffer the consequences. Even the loss of one hour of sleep time that accumulates for several days can have a powerful negative effect on daytime performance, thinking, and mood.

Circadian rhythms refer to the cyclical changes, like fluctuations in body temperature, hormone levels, and sleep that occur over a 24-hour period, driven by the brain's biological “clock.” These internal 24-hour rhythms in physiology and behavior are synchronized to the external physical environment and social/work schedules. Light and darkness are external signals that “set” the biological clock and help determine when we feel the need to wake up or go to sleep. In addition to providing synchronization in time between various rhythms, the circadian clock also helps promote wakefulness.

Our circadian rhythm (or 'body clock') is influenced by melatonin, as it acts as a cue', anticipating sleep. It is also an important physiological regulator of the sleep-wake cycle.

Thus the homeostatic system tends to make us sleepier as time goes on throughout the waking period, regardless of whether it's night or day, while the circadian system tends to keep us awake as long as there is daylight, prompting us to sleep as soon as it becomes dark. Because of the complexity of this interaction, it is generally agreed that sleep quality and restfulness are best when the sleep schedule is regularly synchronized to the internal circadian rhythms and that of the external light-dark cycle, when we try to go to bed and wake up at around the same time each day, even on days off and weekends. Moreover, the circadian system is particularly intolerant of major alterations in sleep and wake schedules, as anyone who has traveled cross-country by plane or worked the graveyard shift can attest.