Sleep is not a homogenous state throughout the night but rather the sleeper goes through a number of different stages of sleep. The stages progress from being relatively light, a period where the brain and body are drifting off and moving toward sleep, through to incredibly heavy periods where the sleeper is in an intensive dream state and their body is effectively paralysed. Understanding the stages of sleep is useful in explaining a number of important aspects of sleep.
The stages of sleep were discovered by scientists when they first hooked sleepers up to an electroencephalograph. This machine reads brain waves and it enabled researchers to ‘look’ into the minds of people while they were asleep. This revealed that while people appear to be in the same state the whole time, they actually go through five distinct stages of sleep.
The first stage of sleep marks the beginning of the process. It is relatively light in comparison to the other stages. In reality it can be seen as a transitional stage between being awake and asleep. In this stage the brain produces theta wages, which are slow brain waves. This stage only lasts between 5-10 minutes.
In the second stage the brain starts to produce bursts of rapid and rhythmic brain wave activity which are called sleep spindles. The body temperature and heart rate begin to slow as well. This stage lasts around 20 minutes.
As the sleep moves into the third stage the brain begins to produce very deep and slow delta brain waves. This stage is the transition between the two lighter levels and the two deeper levels.
Now as they enter into the fourth stage, the brain continues to produce delta brain waves. This stage lasts around 30 minutes and it is during this period that most incidences of sleep walking and bed wetting occur.
The final stage, the fifth, is known as the rapid eye movement stage, for good reason. It is during this stage that we do the most dreaming. This stage is characterised by rapid eye movement, increased respiration and increased brain activity. While the brain is more active than at any other point, the body is generally paralysed, unable to act out the physical movements related to the dream.
Often the sleeper will not go straight through those five stages in sequence but will often go through the first four, then back to 3 then 2 then into stage 5. Most people will go through these cycles 4-5 times a night and we usually enter stage 5 for the first time around 90 minutes after going to sleep.
Waking people in the last three stages will leave them feeling groggy and tired, while being woken in the first two is nowhere near as problematic.
While sleep might look unchanging throughout the night there are actually five different stages a sleeper will go through. Understanding these can help you to work out why you might feel more tired when woken sometimes.
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