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Stages of Sleep

Sleep researchers distinguish between several different phases of sleep. They follow each other in a cycle that is repeated three or four times per night. Each complete cycle takes about an hour and a half. A group of researchers proposed in 1937 that the cycle be divided into 5 stages identifiable in the EEG record, labeled A through E. Now these stages are labeled 1 to 4, with the fifth being REM sleep. Some researchers refer to the wakefulness before sleep as stage 0.

What is stage zero?

During Stage 0, while a person is still awake but sleepy, the EEG shows mostly alpha waves: waves in the 8-12 per second frequency range. Alpha waves are normally blocked by eye movements or focused attention, so the appearance of alpha waves indicates that a person is resting the eyes and letting go of the day's concerns. If alpha waves are present, the individual is still awake and resting quietly.

What is the most distinctive characteristic of sleep onset?

Stage 1 sleep (the earliest stage) marks the onset of sleep. It is characterized by the disappearance of alpha waves and a simultaneous cut-off of attention to the environment. If subjects have their eyes taped open while very sleepy, then are instructed to press a button each time a light flashes, they will suddenly stop pressing the button when they enter Stage 1. The light continues to flash into their still-open eyes, but as soon as they enter Stage 1, they cease to respond to it.

How could researchers tell when somebody fell asleep?

The switch of attention from outside to inside is often accompanied by odd thoughts or imagery that are distinctly dream-like (hypnagogic mentation). At the same time, the alpha waves in the EEG disappear. Independent observers can agree when sleep onset occurs to within 2 minutes of each other 80% of the time using only EEG records.

Soon after a person falls asleep, stage 2 of non-REM sleep occurs. It is identified by two distinctive EEG patterns called sleep spindles and K-complexes. Except for the first one or two cycles of sleep stages, when REM sleep follows stage 4 sleep, REM sleep usually follows stage 2 sleep.

What is "slow-wave" or "delta" sleep? Which stage of sleep is the "deepest" for humans?

Stages 3 and 4 of non-REM sleep are identified by increasingly prominent brain waves on the EEG called delta waves. Delta waves are large, slow waves caused by the synchronized firing of many neurons. The neurons are in a resting pattern, pulsing together. This phase of non-REM sleep is called slow wave sleep or delta sleep. It is the deepest stage of sleep for humans, if deepness is defined by how hard it is to awaken a creature. In cats and other animals, the REM state is the deepest state. In humans, stage 4 non-REM sleep is deepest.

What is a "sound" sleep, and what does this indicate?

People describe a good sleep as sound, which means "solid and unmoving." As the expression suggests, the type of sleep that feels deepest to humans is a sleep with little body movement. Usually this indicates lots of stage 3 or stage 4 sleep. Tossing and turning accompanies lighter sleeps with more time in stages 1 and 2. People often get all the delta sleep they need during the first two sleep cycles, so they remain in lighter stages of sleep after that, often alternating between stage 2 and REM sleep for the remainder of the night.

When does the first true REM sleep period usually occur? How long does it last?

Sleep stages during one night (after Dement & Kleitman, 1957). Note that the REM periods were usually entered from stage 2. The REM periods lasted from 2 to 30 or more minutes.

During REM sleep, an EEG shows an arousal pattern similar to the pattern shown by an awake brain. At the same time, muscle tension below the neck almost disappears. Breathing and heart rate fluctuate.

After the REM sleep period, the sleeper typically returns to stage 2 sleep, and the cycle of non-REM to REM sleep repeats again. A single sleep cycle consisting of non-REM stages followed by an REM period averages 90-100 minutes in length. Typically, three or four REM periods occur each night.

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