The Sleep-wake cycle consists of roughly 8 hours of nocturnal sleep and 16 hours of daytime wakefulness in humans, 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 we wake up, the homeostatic drive for sleep accumulates, reaching its maximum in the late evening when most individuals fall asleep. 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. 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”. In humans, the biological clock consists of a group of neurons in the hypothalamus of the brain called the Suprachiasmatic (SCN). In humans, light is the strongest synchronizing agent. Light and darkness are external signals that “set” the biological clock and help to determine when we feel the need to wake up or go to sleep. It tends to keep us awake as long as there is daylight, promoting us to sleep as soon as it becomes dark.
When we attempt to stay awake against the schedule dictated by our circadian clock, our mental and physical performance is greatly diminished. This results in excessive sleepiness, poor sleep, loss of concentration, poor motor control, slowed reflexes, nausea and irritability. Those who perform shift work, particularly on night shifts, also may experience the effects of a disrupted circadian sleep-wake cycle. Taking a nap in the middle of a night shift may help to maintain or improve alertness, performance and mood.