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"
'via Blog this'
Whatever is the case this is an interesting subject and the BBC piece is well worth a read.