Sleep is an essential part of life.
Humans spend over one-third of their lives asleep. During sleep, the brain undergoes a series of chemical and electrical oscillations. This helps define its different stages and what goes on during a particular stage. Becoming knowledgeable about sleep and its different stages can prove beneficial to the health of any individual.
Types of Sleep:
Sleep is divided into two types, non-REM (non-Rapid Eye Movement) and REM (Rapid Eye Movement). Of these, non-REM sleep divides into 3 distinct stages. Stages 1 through 3 of non-REM sleep is followed by a period of REM sleep in approximately 20-minute intervals (around 90 minutes per non-REM/REM cycle). After a full cycle, the sleep episode goes back to the beginning and this process is repeated. (This basic loop varies as the sleep episode progresses, at times beginning the cycle at stage 2 rather than stage 1).
At the beginning of sleep, stage 3 of non-REM sleep constitutes the majority of the cycle. Meanwhile, the REM period is rather short. As the sleep episode progresses, time spent in the 3rd stage of non-REM sleep shortens while that of REM sleep lengthens. During the last non-REM/REM cycle before waking up, stage 3 non-REM has completely disappeared. During this time, REM sleep is at its longest interval.
Stages of NREM:
Stages 1-3 of non-REM sleep reflect the progression of a sleep episode as it deepens.
Stage 1:
Stage 1 of non-REM sleep is the drowsy stage. During this time environmental stimuli still affect us, like sounds and smells. Hypnagogic hallucinations are possible during stage 1 of non-REM as an individual transitions from wakefulness to sleep. Hypnagogic hallucinations are described as dream-like episodes affected by stimuli of the immediate environment (like light and sounds) and by events happening briefly before sleep onset.
Stage 2:
Stage 2 of non-REM sleep is described as light sleep. An individual in stage 2 non-REM sleeps deeper than someone in stage 1 and requires a greater disturbance to wake. Stage 2 non-REM constitutes the majority of a sleep episode (approximately 45 to 55%) and lengthens with each respective cycle (1).
EEG (electroencephalogram) measures the analysis of electrical activity in the brain during sleep. EEG readings of stage 2 non-REM show two important phenomenon- sleep spindles and K-complexes. Both have an effect on memory consolidation and refreshing one’s learning capacity (1)(2).
Sleep spindles occur more frequently in individuals who are in the learning process (exposed to new information). K-complexes occur in response to environmental stimuli- like noise or light. K-complexes play a role in preventing (cortical) arousal. These help keep you asleep by controlling the release of cortisol in response to stimuli.
Stage 3:
Stage 3 (previously known as stages 3 and 4) of non-REM sleep is considered deep sleep. During this stage, high-voltage slow wave activity on EEG occurs and therefore coined slow-wave-sleep. It is much harder to awaken an individual from this stage when compared to the previous two.
Stage 3 non-REM sleep starts as the lengthiest stage at the beginning of a sleep episode and progressively shortens until it completely disappears. Growth hormone also releases during slow-wave-sleep, contributing to physical recovery. One loses body awareness during this stage and it becomes possible to act out dreams in some cases (a type of sleep disorder).
REM Sleep:
REM sleep is a unique type of sleep whose electrical readings resemble those of wakefulness. During REM sleep, the brain remains active and body movement restricted to prevent the acting out of vivid dreams. The body becomes paralyzed with the exemption of structures in the thorax and respiratory tract necessary for breathing. The eyes too are free to move.
Most dreams that we can remember happen during REM sleep. The active state of REM sleep allows for individuals to experience dreams that are more realistic and memorable. A trick for remembering dreams more vividly (for interpretation and the like) is to set an alarm 10 to 15 minutes before the usual wake-up time. By doing this you are pulling yourself directly out of REM sleep and are therefore more likely to recall details.
References:
“Sleep Physiology.” Sleep Disorders and Sleep Deprivation: an Unmet Public Health Problem, by Harvey R. Colten and Bruce M. Altevogt, Institute of Medicine, 2006, pp. 34–39.
Laventure, Samuel, et al. “NREM2 And Sleep Spindles Are Instrumental to the Consolidation of Motor Sequence Memories.” PLOS Medicine, Public Library of Science, 31 Mar. 2016, journals.plos.org/plosbiology/article?id=10.1371%2Fjournal.pbio.1002429.
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