Sleep Disorders

11 December 2014

What happens when you fall asleep?

When you fall asleep, you progress through five stages of sleep with the last phase being the rapid eye movement (REM) phase, after which the cycle begins again.


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.

Therefore the more REM sleep your child gets the better. As your baby grows and the brain develops less time is spent in REM sleep.

Read: Why shut-eye is important

What happens during REM sleep?

During REM sleep your brain rhythms look similar to those when you’re awake. Your eyes jerk rapidly in various directions, breathing becomes more rapid and blood pressure rises. But because messages from the brain stem to the rest of the body are ‘‘switched off ’’ your muscles become temporarily paralysed. This is essential – imagine what would happen if you acted out your dreams! Despite the temporary paralysis men may still experience erections.

It’s difficult to wake you from REM sleep and if it happened you would be disoriented and your thoughts bizarre. This is also when you’d be dreaming about that giant hairy worm chasing you and a scantily clad Angelina Jolie (or Brad Pitt) over the lip of a volcano – before you simply fly away.

REM sleep is an extremely sensitive state, affected by food and drink (particularly caffeine), medication (asthma treatments, for example), alcohol, diet pills (which contain stimulants), diuretics, cigarettes and extreme cold or hot environments.

If REM sleep is interrupted just once during the night your body doesn’t return to the normal sleep cycle when you go back to sleep – it goes directly back into REM sleep to try to catch up. Mothers of young children spend more time in REM sleep than in any other phase because of the constant interruptions of their sleep cycle. After REM sleep the entire sleep cycle starts again.

Sleep has recurring cycles – between three and five a night – of 90 to 110 minutes each. The cycles of sleep become more shallow as the night progresses.

Deep sleep is also the phase when hormones such as testosterone and growth hormones are secreted. In adults growth hormones ensure cells, skin, bone tissue and muscles remain healthy – your proverbial beauty sleep. In babies and children they facilitate growth and trigger puberty.

Read more:
The importance of dreams
Just how much sleep is enough?
What are sleep disorders?

Compiled by Mari Hudson and Elise-Marie Tancred (Reviewed in January 2012)


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Sleep disorders expert

Dr Alison Bentley is a general practitioner who has consulted in sleep medicine and sleep disorders, in both adults and children of all ages, for almost 30 years. She also researches and publishes on a number of sleep-related topics both in formal research journals and lay publications including as editor of Sleep Matters, an educational newsletter on sleep disorders for doctors.

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