Tuesday, 17 February 2015

Stages Of Sleep

Usually sleepers pass through five stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages progress cyclically from 1 through REM then begin again with stage 1. A complete sleep cycle takes an average of 90 to 110 minutes. The first sleep cycles each night have relatively short REM sleeps and long periods of deep sleep but later in the night, REM periods lengthen and deep sleep time decreases.
Stage 1
Stage 1 is light sleep where you drift in and out of sleep and can be awakened easily. In this stage, the eyes move slowly and muscle activity slows. During this stage, many people experience sudden muscle contractions preceded by a sensation of falling. 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 weren't really asleep.
Stage 2
In stage 2, eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves. 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.
Stage 3
When a person enters stage 3, extremely slow brain waves called delta waves are interspersed with smaller, faster waves. This stage was previously divided into stages three and four. Deep, slow brain waves known as delta waves begin to emerge during stage 3 sleep. This stage is sometimes referred to as delta sleep because of the slow brain waves known as delta waves that occur during this time. 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. Bed-wetting and sleepwalking are most likely to occur at the end of this stage of sleep.
Stage 4
In stage 4, the brain produces delta waves almost exclusively. Stages 3 and 4 are referred to as deep sleep or delta sleep, and it is very difficult to wake someone from them. In deep sleep, there is no eye movement or muscle activity. This is when some children experience bedwetting, sleepwalking or night terrors. 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. 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 because of increased brain activity, but voluntary muscles become paralyzed. In 2008 the sleep profession in the US eliminated the use of stage 4. Stages 3 and 4 are now considered stage 3.
Slow wave sleep comes mostly in the first half of the night, REM in the second half.  Waking may occur after REM.  If the waking period is long enough, the person may remember it the next morning.  Short awakenings may disappear with amnesia.
In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, males develop erections and the body loses some of the ability to regulate its temperature. This is the time when most dreams occur, and, if awoken during REM sleep, a person can remember the dreams. Most people experience three to five intervals of REM sleep each night.
Infants spend almost 50% of their time in REM sleep. Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM sleep.

As sleep research is still a relatively young field, scientists did not discover REM sleep until 1953 when new machines were developed to monitor brain activity. Before this discovery it was believed that most brain activity ceased during sleep. Since then, scientists have also disproved the idea that deprivation of REM sleep can lead to insanity and have found that lack of REM sleep can alleviate clinical depression although they do not know why. Recent theories link REM sleep to learning and memory.


Stage
Frequency
Amplitude (micro volts)
Waveform type
awake
15-50
<50
pre-sleep
8-12
50
alpha rhthym
1
4-8
50-100
theta
2
4-15
50-150
splindle waves
3
2-4
100-150
spindle waves and slow waves
4
0.5-2
100-200
slow waves and delta waves
REM
15-30
<50

The brain waveform during REM has low amplitudes and high frequencies, just like the waking state. Early researchers actually called it "paradoxial sleep".
The American Academy of Sleep Medicine has floated use of the term "Stage R" for REM sleep, but this new terminology has not caught on.


Stages
Waking
REM Sleep
NREM Sleep
Stage 0
Stage R
Light Sleep
Deep Sleep
Stage 1
Stage 2
Stage 3
Stage 4
Eyes open, responsive to external stimuli, can hold intelligible conversation
Brain waves similar to waking.  Most vivid dreams happen in this stage.  Body does not move.
Transition between waking and sleep.  If awakened, person will claim was never asleep.
Main body of light sleep.  Memory consolidation.  Synaptic pruning.
Slow waves on EEG readings.
Slow waves on EEG readings.
16 to 18 hours per day
90 to 120 min/night
4 to 7 hours per night