An electroencephalograph (EEG) enables scientists to distinguish five phases in each cycle. The phases of sleep progress in a cycle from phase one to REM (rapid eye movement) sleep, after which the cycle starts again.
PHASE ONE is light sleep, when you’re still half awake. Your muscles relax, your pulse slows and your eyes move from corner to corner. This phase lasts just a few minutes. The slightest disturbance will wake you and you may experience a sensation of falling. That’s because your muscles have started to relax.
PHASE TWO You spend almost half of your total sleep time in phase two, called true sleep. It lasts for almost 20 minutes at a time and your heartbeat, breathing and brain waves slow down.
PHASE THREE is the start of deep sleep. You’re now thoroughly relaxed. In this phase huge, slow brain waves called delta waves begin to occur. Your breathing and heartbeat decrease to their lowest possible levels.
PHASE FOUR The brain produces almost only delta waves and it’s very difficult to wake you. If you’re disturbed during deep sleep you’ll struggle to adjust to being awake and will feel disoriented and groggy. Deep sleep is the phase during which some children experience bedwetting, night terrors or sleepwalking.
PHASE FIVE The last phase of the cycle is REM sleep. It starts about 70 to 90 minutes after you’ve fallen asleep. Adults spend about 20 per cent of their total sleep time in this state while for babies it’s about 50 per cent. REM sleep lasts longest at night. If you doze off during the day the REM phase is longer during morning naps than in the afternoon. REM sleep may play an important role in brain development.