Five stages of sleep
If our October feature on sleep woke you up to its many mysteries, here are some more fascinating facts about the land of nod…
Normal sleep has distinct phases that are repeated several times a night:
The beginnings of sleep
During the early phase of sleep, you are still relatively awake and alert. Your brain produces short, fast electrical pulses that are known as beta waves. But as we drop off, the brain slows, producing bigger alpha waves.
This stage only lasts five to ten minutes, during which the brain produces very slow brain waves called theta waves.
This part of sleep lasts for approximately 20 minutes. As we slip onto unconsciousness, ‘sleep spindles’ appear. These indicate short bursts of intense electrical activity in the brain. Body temperature starts to decrease and heart rate begins to slow.
This is the time when we start to move between light sleep and deep sleep. The brain produces big, slow delta waves.
This is the super-restful deep sleep or ‘slow wave sleep’. For around 30 minutes rolling brainwaves are produced as many areas of the brain act together in a co-ordinated but slow fashion. This is when the day’s learning and memories are consolidated and sleepwalking is most likely to occur.
This stage is known as REM or rapid eye movement sleep, and we normally reach it after 90 minutes of sleep. This is a very deep sleep, when we have our most emotional and bizarre dreams, but the brain is very active. While our body is more relaxed and our voluntary muscles become paralysed, our eyes start to dart about under our closed lids and we breathe more rapidly. We have several episodes of REM sleep each night. The first may last only ten minutes, but each episode lasts longer. By the fifth sleep cycle, at the end of sleep, it can last up to an hour.
The sequence of sleep stages
Sleep doesn’t simply progress from stage 1 to 5 in sequence. What happens is that after you have got to stage 4, or deep sleep, for the first time, you go back through stage 3 and 2 again, before entering stage 5, or REM sleep. After REM is over, the body normally returns to stage 2 again. If you sleep seven to eight hours you will experience four or five full sleep cycles. If you choose to lie in, your well-rested body won’t need more deep sleep, so you’ll go straight to REM instead, which is why dreams may seem particularly vivid in the morning.