Sleep is the process in which humans and other animals periodically rest, with greater or lesser degrees of unconsciousness and decreased responsiveness to the surrounding world. Sleep occurs cyclically, roughly every 24 hours even though the average human inner body clock usually runs a 24.5-25.5 hour cycle. This cycle gets reset daily (to match 24 hours) with various stimuli such as sunlight. One of the correlates of this cycle is the level of melatonin, which is high at times when we tend to sleep.
We progress through a series of stages as we sleep. It was the invention of the electroencephalograph (EEG) that allowed scientists to study sleep.
During the 1950s, a graduate student named Eugene Aserinsky used this tool to discover what is known today as REM sleep.
There are two main types of sleep:
1. Non-Rapid Eye Movement (NREM) Sleep (also known as quiet sleep)
2. Rapid Eye Movement (REM) Sleep, (also known as active sleep or paradoxical sleep)
During the earliest phases of sleep, you are still relatively awake and alert. The brain produces what are known as beta waves, which are small and fast.
The Stages of Sleep
As the brain begins to relax and slow down, slower waves known as alpha waves are produced. During this time when you are not quite asleep, you may experience strange and extremely vivid sensations known as hypnagogic hallucinations. Common examples of this phenomenon include feeling like you are falling or hearing someone call your name.
There are 3 NREM stages and a REM stage of sleep.
NREM Stage 1
Stage 1 is the beginning of the sleep cycle, and is a relatively light stage of sleep. Stage 1 can be considered a transition period between wakefulness and sleep.
In Stage 1, the brain produces high amplitude theta waves, which are very slow brain waves.
This period of sleep lasts only a brief time (around 5-10 minutes). If you awaken someone during this stage, they might report that they were not really asleep.
NREM Stage 2>People become less aware of their surroundings
Stage 2 is the second stage of sleep and lasts for approximately 20 minutes. The brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. Body temperature starts to decrease and heart rate begins to slow. According to the American Sleep Foundation, people spend approximately 50 percent of their total sleep in this stage.
NREM Stage 3>Muscles relax
During this stage, people become less responsive and noises and activity in the environment may fail to generate a response. It also acts as a transitional period between light sleep and a very deep sleep.
Older studies suggested that bed-wetting was most likely to occur during this deep stage of sleep, but some more recent evidence suggests that such bed-wetting can also occur at other stages. Sleepwalking also tends to occur most often during the deep sleep of this stage.
During REM sleep:>The brain becomes more active
Most dreaming occurs during the fourth stage of sleep, known as rapid eye movement (REM) sleep. REM sleep is characterized by eye movement, increased respiration rate and increased brain activity. The American Sleep Foundation suggests that people spend approximately 20 percent of their total sleep in this stage.
The Sequence of Sleep Stages
It is important to realize that sleep does not progress through these stages in sequence. Sleep begins in stage 1 and progresses into stages 2, and 3. After stage 3 sleep, stage 2 sleep is repeated before entering REM sleep. Once REM sleep is over, the body usually returns to stage 2 sleep. Sleep cycles through these stages vacillate approximately four or five times throughout the night.
On average, we enter the REM stage approximately 90 minutes after falling asleep. The first cycle of REM sleep might last only a short amount of time, but each cycle becomes longer. REM sleep can last up to an hour as sleep progresses.
While sleeping is considered to be a passive activity, in most stages, the brain is quite active.
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