Many people believe that sleep is a passive, dormant part of daily life, but in fact, sleep is a very active state. Our brains are highly engaged during sleep, even more varied than when we are awake.
Before the 1950s it was commonly thought by scientists that sleep caused brain activity to completely stop during sleep. This was believed up until the invention of the electroencephalogram (EEG), which allowed the discovery of REM (Rapid Eye Movement) sleep. This machine is able to measure brain wave activity and maps the electrical activity from one area of the brain.
There are two main types of sleep:
- Non-Rapid Eye Movement (NREM) Sleep
This occurs during the first 90 minutes and restores the physical body, releases growth hormones, and stimulates the immune system to defend against infections
- Rapid Eye Movement (REM) Sleep
This occurs after REM sleep and restores the brain and captures memories which allow learning to process.
Usually, during sleep you can pass through 5 phases: 1, 2, 3, 4, and REM (Rapid Eye Movement).
Stage 1 is considered a ‘Light sleep’ which lasts around 10 minutes. This stage is where you are in a state of calmness, you drift in and out of sleep and can be awoken easily. During Stage 1 the eyes move slowly and muscle activity slows down. In this stage, it is common for people to experience sudden muscle contractions called hypnic myoclonia, which is followed by a sensation of falling.
This second sleep stage usually lasts around 30 to 45 minutes. During this stage, your movement drops and you become oblivious to your surroundings. Eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves called ‘sleep spindles’.
Stage 3 and 4
These stages are known as ‘deep sleep’, it is very difficult to wake someone from this without them acting disorientated and groggy. Stage 3 is only very brief and is a transitional period between light and deep sleep, this is when slow brain waves begin called delta waves. Stage 4 is a state of complete relaxation and lasts for approximately 30 minutes; there is no eye movement or muscle activity during this time.
The first REM sleep period usually occurs about 70 to 90 minutes after we fall asleep. During this time breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles become temporarily paralysed. The brain waves in this stage are increased to levels usually seen when a person is awake, and this is the time when dreams are most likely to occur. Most people experience REM sleep three to five intervals a night.
Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM sleep, whilst infants spend almost 50% of their time in REM sleep.
These stages can pass regularly from stage 1 through to REM, and begin at stage 1 again. This complete cycle lasts roughly 90 to 110 minutes and these cycles can occur multiple times throughout the average sleep. A person will cycle through all the stages of sleep four or five times on average during an eight-hour rest period.
During the start of your sleep the cycles with have shorter REM periods and longer periods of ‘deep sleep’ . However, later in the night this cycle reverses and REM sleep periods increase in length, by the morning you spend most of your sleep in stages 1, 2 and REM.