Why do we need to sleep?
The most widely accepted theory about sleep postulates that we fall into this state for two reasons: to conserve energy and to recuperate from the previous day’s activities. Let’s take them one by one .
First, we conserve energy when we sleep because our metabolic rate the rate at which our bodies use energy to function is reduced by at least 25 percent over daytime levels. Oxygen consumption, heart rate, and body temperature all decline during the first few hours of sleep and reach an all time low about an hour before we wake up.
In essence, then, sleep allows the body to maintain homeostasis a relatively stable internal environment. As for the recuperative and restorative powers of sleep, it appears that the whole body, including the central nervous system, needs to slow down in order to repair itself.
We know that more than half of the total daily output of human growth hormone1 a substance released by the pituitary and integral to growth in children and tissue repair in adults, is released during the first few hours of sleep.
At the same time, cortisol and other corticoids, known to stimulate heart and respiration rates, fall to their lowest levels during sleep. Many sleep experts, however, believe that it is the brain and not the body that benefits most from sleep.
Indeed, studies have shown that sleep deprivation results in far more psychological than physical deficits our concentration falters and our mood darkens far more quickly than our bodies experience any lasting physical impairments.
In fact, people with insomnia or other sleep problems are more likely to develop psychiatric illnesses than their sleep sated peers. The truth is, however, sleep researchers still don’t know with any certainty the specific biological functions of sleep.
Indeed, people have been kept awake in experiments for as long as eleven days straight with no discernible physiological damage and only minor changes in circadian hormonal rhythms. On
the other hand, no one can deny that a lack of sleep makes us at least feel bad our mental performance and memory tend to suffer, we become irritable and/or anxious, and our physical reflexes falter, however temporarily.
In the end, then, sleep is good for us, body, mind, and soul.
What happens during sleep?
Sleep is composed of two distinct physiological states, as different from each other in some ways as each one is from wakefulness. They are known as rapid eye movement (REM) and non-REM sleep and we pass through about four to six non REM/REM sleep cycles every night. Let’s look at each stage separately:
As people fall asleep, they progress through the non-REM stages and, then, about ninety minutes later, they have their first episode of REM sleep. As the night progresses1 the episodes of non-REM sleep become shorter and those of REM sleep longer. Most slow-wave sleep occurs during the first third of the night, and most REM sleep during the last third.