A sleep cycle takes 90 mins. The stages alternate between REM (rapid eye movement) and NREM (non-rapid eye movement).
As we fall asleep, we enter NREM, accounting for 75 per cent of a night’s sleep. This comprises:
N1: This is where you’re between being awake and falling asleep – light sleep.
N2: The onset of sleep; you become disengaged from your surroundings. Breathing and heart rate are regular; body temperature drops.
N3: The deepest, most restorative sleep, blood pressure drops; breathing is slower; muscles relax; blood supply to your muscles increases; tissue repair occurs.
REM accounts for the other 25 per cent; it begins about 90 mins after you fall asleep and recurs roughly every 90 minutes thereafter; the periods grow longer later in the night. This period supports optimum daytime performance and is when the brain is most active (most dreams occur), and your body is immobile as your muscles are ‘ turned off’.