While sleep is still largely an uncharted zone, a few things are known about the normal human sleep cycle:
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The sleep cycle is about 90 minutes long
The sleep cycle is composed of five stages
stage one - drowsiness & light dreams
stage two - light sleep
stages three and four - Delta or deep sleep
stage five - REM or deep dream sleep
The composition of total sleep time varies greatly between normal sleep and sleep disturbed by a sleep disorder:
normal sleepabnormal sleep
stage 1 5% 50%
stage 2 50% 30-35%
Delta 20% 10-15%
REM 25% 0-5%
This drop in the amount of time spent in the deep sleep stages of Delta and REM sleep is significant and is the reason why people with a sleep disorder feel tired.
They do not yet know why a lack of REM sleep leaves you tired, but they do know that during Delta sleep, the body restores muscle, tissue, bone and blood cells that have been damaged, destroyed or died off during the course of the day and releases and /or produces hormones that regulate many of the vital functions of the body. If this repair, replacement, restoration and replacement process does not take place, it weakens the immune system, and as a result, over time, a lack of deep sleep leaves you more prone to getting sick.
The sleep cycle repeats 4-6 times a night
We don't necessarily always progress through the sleep stages in a linear fashion. If your sleep is normal, you may reach your first REMcycle 60 to 90 minutes after you fall asleep, having gone from stage 1 to 2 to Delta to REM. This Delta may last 10 to 15 minutes and this REM may be very short. In your next cycle you may skip stage 1 entirely and go right to stage 2 then a slightly longer Delta and another short REM, etc. In normal sleep, Deltas are longer during the first third or half of the night and REMs are longer during the last half of the night, particularly between 3 and 6 am.
In abnormal sleep, some people never get into REM. However, if you do get into REM sleep, you will dream, although you may not remember the dreams. If you happen to wake up during a REM stage or right after one, you will most likely remember your dream, but unless you focus on it or write it down, by morning, the dream that seemed so vivid the night before will most likely have faded from your memory.
In sleep disrupted by a sleep disorder, there may be two to three cycles cycles before Delta and/or REM sleep is reached. The more severe the daytime sleepiness, the more likely it is that REM is never achieved. Certain medications have been known to have these effects, known as delayed onset of Delta or REM sleep or REM suppression. As mentioned previously, without Delta or REM, sleep is pretty much a waste of time. To learn how to maximize your Delta and REM sleep, read The Guide to Better Sleep Naturally.