The Stages of Sleep
Welcome to our page on the stages of sleep.
The average human being spends about one third of their life asleep which is roughly about the same time spent at work. So, taking into account the old adage that you live “three score and ten” (70), means you will have spent twenty of those seventy years asleep and five of them dreaming. Interestingly, and keeping in mind the amount of time spent in the aforementioned states, researchers really have little understanding about sleep or its purpose.
Different Sleep Stages
What is known about sleep is that we absolutely need it and if we don’t get it in terms of both quality and quantity, problems arise in our waking hours. There are two clear-cut states of sleep: non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). These two states occur in 90 minute cycles. Each cycle is repeated 5 to 6 times a night and includes four stages of NREM and REM.
Typically, REM sleep is a by-product of NREM sleep. As we fall asleep, we enter the period known as Stage 1; the first of our "sleep cycles" lasts between 1-5 minutes. Soon after, approximately a few minutes, we adjust into our "baseline" sleep; this is Stage 2.
Stage 2 sleep occupies approximately 50-65% of our sleep time. Within 15-20 minutes we evolve into Stage 3 and then Stage 4. These stages are known as “delta sleep” or “slow wave sleep” because of the very high voltage and slow brain waves. Delta sleep lasts approximately 15-30 minutes and is the "deepest" stage of sleep and the therefore the most restorative. It is this type of sleep yearned for by sleep deprived individuals and is the “dead” sleep one sees in children whereby nothing seems to wake them.
As we pass through these first four stages of sleep our respiration and heart rate slow and the body is almost immobile. Then, suddenly after 20-30 minutes of slow wave sleep, we engage in very active brain wave pattern known as REM sleep. Concurrent with this leap into REM, our respiration and heart rate increase considerably and we lose our ability to use our postural or skeletal muscles. In other words, we become physically paralysed.
As we engage in REM sleep, our brain becomes highly active leading to dreams. Just as when we are awake, our eyes move down to midline and begin to move sporadically as if following the goings on of the environment. This REM period will last 10-20 minutes and then we transfer back to Stage 2 again, thus, ending one sleep cycle. The whole process starts again, however as the cycles persist, delta sleep becomes less frequent and Stage 2 and REM sleep is extended. If we engage in a “full night’s sleep”, the final cycle will be made up of 50% Stage 2 sleep and 50% REM sleep. This why it is said most dreaming occurs just before the sleeper awakes.
Visit our REM sleep and dreaming page for more on the science behind REM sleep.
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