Sleep

What is sleep? Sleep is a state of altered consciousness and not a turning off of the brain. Although sleep appears to be the opposite of wakefulness, these two stages have a lot in common. Thinking and memory are active in both states, and a sleeping person is sensitive to their crying baby. Studies of sleeping deprivation have shown that sleep is a fundamental need that human cannot do without it. The brain needs periods of sleep in order to rest repair itself and process information received during the period of wakefulness.

The brain produces electrical impulses or brain waves which can be detected using electroencephalography (ECG). Studies using EEG from the 1950’s onwards showed from the patterns of brain waves that sleep can be divided into two distinct stages which alternate in cycles of around 90 minutes throughout the 8 hours sleep period. The chart below shows how a sleeper passes repeatedly through these two states, about every 90 minutes, during an 8 hours sleep period.
The first state is non rapid eye movement sleep (NREM) which takes up about 80 percent of the sleep period, during NREM sleep, the sleeper becomes relaxed and drifts into deeper and deeper sleep. The depth of NREM sleep is measured in four stages of progressively greater depth of sleep. Stage 4 is the deepest when body metabolism is lowest and brain activity least. Dreaming does not occur during NREM sleep.

After 60- 90 minutes, the second stage of sleep begins. The brain becomes more active with wave patterns resembling those of an awake person, temperature and blood flow increase, and the eyes move around beneath the closed eyelids, giving this sleeping state its name of rapid eye movement (REM) sleep. Dreaming takes place during REM state of sleep.
The first REM period lasts for about 5-10 minutes but as the chart shows, the length of REM periods increases as sleep continues. Deep NREM sleep is believed to be the time when the brain rests and repairs itself; REM sleep is when the brain analyzes the day’s events and sorts out emotional problems some researchers believe REM sleep not to be Sleep at all but a third state of existence in addition to sleep and wakefulness.