Sleep is a time of bliss that many people should not take for granted, but still do. “Oh, I’ll just stay up past midnight finishing a big project assigned 3 weeks ago and cram in 2 hours of sleep from 4 to 6 am,” says all the students while cracking their third can of cold Celsius. The point is, sacrificing precious hours of sleep does more harm than good in retrospect. During sleep, our brains work in overdrive not only to cleanse the toxins accumulated throughout the day, but also to consolidate neural pathways that enhance memory.
The 4 stages of sleep can be classified into two types: non-rapid eye movement (non-REM) sleep and rapid eye movement (REM) sleep. Non-REM sleep occurs in 3 stages in ascending order that eventually culminates into one stage of REM sleep. The body cycles through both non-REM and REM sleep throughout a typical night of sleep, with the period of REM sleep increasing after each sleep cycle. An average sleep cycle lasts around 90 minutes, so the body should aim for between 4-6 complete cycles of sleep per night to wake up feeling refreshed.
Stage 1 of non-REM sleep lasts for only a few minutes. In the first stage of sleep, the body reflexively relaxes as heart rate and breathing slows. Muscles and brainwaves also begin to relax as opposed to daytime hours. Hypnic jerks, or the sensation of suddenly twitching and jolting awake, also occur during stage 1 of non-REM sleep.
In stage 2 of non-REM sleep, the body’s heart rate, breathing, muscles, and brainwaves further relax. Eye movements cease, and awakening from external stimuli becomes much harder at stage 2. The body spends most of its time in stage 2 of non-REM sleep, as per Image 1 below.
Stage 3 of non-REM sleep occurs right before the vivid dreaming that happens in REM sleep. In stage 3 of non-REM sleep, the body’s biological activities reach a trough, and it becomes even more difficult to awaken compared to stage 2.
REM sleep happens after all 3 stages of non-REM sleep, and this stage of sleep is accompanied by biological activities akin to being awake. In REM sleep, the eyes rapidly dart in all directions as dreams manifest. Heart rate, blood pressure, and breathing all increase during REM sleep.
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