Scientists still don’t know exactly why we sleep, although sleep seems to be essential for our health. So what happens after you close your eyes and drift off to sleep?
Rather than being one uniform state, sleep has 2 distinct phases: rapid eye movement (REM) sleep and non rapid eye movement (non-REM) sleep. Non-REM sleep can be broken down into 4 gradually merging stages. When scientists research sleep, they often look at the brain’s electrical activity, measured with an electroencephalogram (EEG). The different stages of sleep have different features and look different on an EEG, as shown in the table below.
|The stages of sleep|
|Sleep stage||Features||Electroencephalogram (EEG) trace|
|Stage 1 non-REM||Light sleep||
|Stage 2 non-REM||Light sleep||
|Stage 3 non-REM||Deep sleep||
|Stage 4 non-REM||Deep sleep||
|(REM = rapid eye movement)|
The diagram below shows the sequence of sleep stages that a person would typically go through during a night.
REM sleep happens about every 90 minutes during the night, and REM periods lengthen as the night progresses while the time spent in non-REM stages 3 and 4 decreases.
Over a typical night, adults spend about 20 per cent of the time in REM sleep and 80 per cent of the time in non-REM sleep. Infants spend at least half of the time they’re asleep in REM sleep, but this proportion decreases as children mature; from late childhood onwards only 20 per cent of the night is spent in REM sleep.
The benefits of sleep
A good night’s sleep makes us feel refreshed and alert. On the other hand, miss out on a few nights’ sleep and chances are you’ll feel below par — research has shown that your attention, learning and physical performance all suffer. These changes can be quite subtle and not necessarily recognised by the person concerned. REM sleep in particular has been linked to learning and to creating new memories.
The benefits of sleep are not just mental. Scientists believe that sleep helps to:
- keep the heart and blood vessels healthy;
- repair the body’s tissues and stimulate growth in children — growth hormone released during sleep is responsible for both these processes;
- strengthen the immune system; and
- regulate appetite and weight and control blood glucose levels.
How much sleep is enough?
The need for sleep varies from person to person. Some people need at least 9 hours of sleep while others feel fine on less than 7. If you’re feeling sleepy during the day, you may not be getting enough sleep. After several nights of getting less sleep than you need, your body builds up a ‘sleep debt’ that you have to ‘repay’ by sleeping longer than usual.
Your sleep requirement changes throughout your life — newborns sleep 16–18 hours a day and pre-school children sleep 10–12 hours. Older children and adolescents need at least 9 hours’ sleep a night, although you may have trouble convincing them of this!
Sleep and ageing
As we pass from young adulthood to midlife, the time we spend in deep sleep (non-REM stages 3 and 4) falls and the time we spend in light sleep (non-REM stages 1 and 2) increases. And as we age further our sleep tends to be interrupted by periods of wakefulness, meaning that we lose increasing amounts of both REM sleep and light sleep. Elderly people also fall asleep earlier in the evening than younger people and wake up correspondingly earlier.
Although many elderly people have sleep difficulties, these are not necessarily a normal part of ageing. Insomnia may be due to medical conditions or medicines that disrupt sleep, so if you’re having trouble sleeping — at any age — it’s best to see your doctor for a thorough evaluation.