Everyone has different beliefs about how much sleep we need but the truth of the matter is that we are all different and therefore all need varying amounts of sleep to not be sleepy in the daytime. Is it a case that we need a full 8 hours or a length of time that is a multiple of 90 minutes?
When we sleep, a recurring cycle averaging 90 minutes occurs and is divided into two categories; non-REM (rapid eye movement) and REM sleep. The cycle will tend to be 65 minutes of normal, or non-REM sleep; 20 minutes of REM sleep (in which we dream); and a final 5 minutes of non-REM sleep. The key factor to a great night sleep is the number of these complete sleep cycles we enjoy.
Non-REM sleep can be divided into four phases; stage one is the light sleep where we are half awake and half asleep, stage two is true sleep the largest part of human sleep, stages three and four are deep sleep where the brain produces delta waves and there is limited muscle activity. The deep sleep stage is where some children experience bed-wetting, night terrors or sleepwalking.
REM sleep usually begins about 70 to 90 minutes after we fall asleep. We have around three to five REM episodes a night. At this stage the brain is extremely active, more so than when we are awake, our eyes dart around (hence the name) and our breathing rate and blood pressure rise. However, our bodies are effectively paralysed, said to be nature's way of preventing us from acting out our dreams.
After REM sleep, the whole cycle begins again.
If we were to sleep completely naturally, with no alarm clocks or other sleep disturbances, we would wake up, on the average, after a multiple of 90 minutes–for example, after 4 1/2 hours, 6 hours, 7
1/2 hours, or 9 hours, but not after 7 or 8 hours, which are not multiples of 90 minutes.
In the period between cycles we are not actually sleeping: it is a sort of twilight zone from which, if we are not disturbed (by light, cold, a full bladder, noise), we move into another 90-minute cycle.
A person who sleeps only four cycles (6 hours) will feel more rested than someone who has slept for 8 to 10 hours but who has not been allowed to complete any one cycle because of being awakened before it was completed.
Facts from BBC – Science & Nature
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