(During the sleep....)
“you cycle through five separate stages of sleep every 90 to 110 minutes, experiencing between three and five dream periods each night”
Sleep is common to mammals, birds and reptiles and has been conserved through evolution, even though it prevents us from performing other useful tasks, such as eating, reproducing and raising young. It is as important as food for keeping us alive; without it, rats will die within two or three weeks – the same amount of time that it takes to die from starvation.
Sleep can be divided into two broad stages, non-rapid eye movement (NREM), and rapid eye movement (REM) sleep. The vast majority of our sleep (around 75 to 80 per cent) is NREM, characterised by electrical patterns in the brain known as ‘sleep spindles’ and high, slow delta waves. This is the time we sleep the deepest.
Without NREM sleep, our ability to form declarative memories, like learning to associate pairs of words, is seriously impaired; deep sleep is important for transferring short-term memories into long-term storage. Deep sleep is also the time of peak growth hormone release in the body, which is important for cell reproduction and repair.