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
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)