http://www.bbc.co.uk/news/magazine-16964783
Stages of sleep
Every 60-100 minutes we go through a cycle of four stages of sleep
Stage 1 is a drowsy, relaxed state between being awake and sleeping – breathing slows, muscles relax, heart rate drops
Stage 2 is slightly deeper sleep – you may feel awake and this means that, on many nights, you may be asleep and not know it
Stage 3 and Stage 4, or Deep Sleep – it is very hard to wake up from Deep Sleep because this is when there is the lowest amount of activity in your body
After Deep Sleep, we go back to Stage 2 for a few minutes, and then enter Dream Sleep – also called REM (rapid eye movement) sleep – which, as its name suggests, is when you dream
In a full sleep cycle, a person goes through all the stages of sleep from one to four, then back down through stages three and two, before entering dream sleep Source: Gregg Jacobs
