Describe the human sleep cycle, including the four stages and REM sleep, and what happens in each.

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 five to 10 minutes).5  If you awaken someone during this stage, they might report that they were not really asleep.

NREM Stage 2

Stage 2 is the second stage of sleep and lasts for approximately 20 minutes. During stage 2 sleep:

You become less aware of your surroundings.

Body temperature drops.

Breathing and heart rate become more regular.

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% of their total sleep in this stage.5

NREM Stage 3

During stage 3 sleep:

Muscles relax.

Blood pressure and breathing rate drop.

Deepest sleep occurs.

This stage was previously divided into stages 3 and 4. Deep, slow brain waves known as delta waves begin to emerge during stage 3 sleep. This stage is also sometimes referred to as delta sleep.

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.6

REM Sleep

During REM sleep:

The brain becomes more active.

The body becomes relaxed and immobilized.

Dreams occur.

Eyes move rapidly.

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% of their total sleep in this stage.



REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due to increased brain activity, but voluntary

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answer: proteins

explanation: complement is   system of circulate in the blood in a inactive form

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