Why do we sleep?
Scientists don’t know why we sleep, although it does seem to be essential for our health. So what happens after you close your eyes and drift off?
Sleep has two phases: rapid eye movement (REM) sleep and non rapid eye movement (non-REM) sleep. Over a typical night, adults spend about 20% of the time in REM sleep and 80% of the time in non-REM sleep. Infants spend at least half of the time they’re asleep in REM sleep. REM sleep is linked to learning and creating new memories.
Non-REM sleep can be broken down into four 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 at the bottom of the page.
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 shows your attention, learning and physical performance all suffer. 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.
Sleep also 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 – some people need at least nine hours of sleep while others feel fine on less than seven. Generally, the younger you are the more sleep you need, and the older you are, the less you need. A good indication that you may not be getting enough sleep is if you feel tired during the day.
Your sleep requirement changes throughout life – newborn babies sleep 16-18 hours a day and pre-school children sleep 10-12 hours. Older children and adolescents need at least nine 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. As we age our sleep tends to be interrupted by periods of wakefulness, meaning we lose increasing amounts of REM sleep and light sleep.
Elderly people also fall asleep earlier in the evening than younger people, and wake correspondingly earlier. And 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.
Types of sleep problems
Insomnia
Not sleeping (insomnia) is not a disease but a problem which most people experience at some time in their lives. It is usually due to a temporary inability to ‘switch off’ the brain. Worry, excitement, anger and pain are all things that can cause this to happen.
Snoring and sleep apnoea
When sleeping, the muscles in the throat relax. In snorers, the muscles relax so much they cause blockages in the airways. This can cause the tongue, soft palate, uvula and tonsils to vibrate, and produce snoring sounds. Many people who have a severe snoring problem may have sleep apnoea, a condition where they stop breathing for a very short time every few minutes during sleep, due to their airway being blocked by their tongue and throat. Their brain constantly wakes them to start breathing again, but they are unaware of the constant sleep/wake pattern during the night. These people may wake up feeling tired and remain sleepy during the day. This problem is common in overweight males and in Maori and Pacific Island men.
Snoring and sleep apnoea will often contribute tofatigue, headaches, lack of concentration, memory loss and mood swings. People with OSA also have an increased risk of high blood pressure, heart disease and stroke.
Other sleep disorders
Delayed Sleep Phase Syndrome (DSPS)
DSPS is when people have a great deal of trouble falling asleep. It is believed to be a disorder of the body’s timing system – the biological clock. Teenagers are particularly susceptible.
Restless Legs Syndrome (RLS) and Period Limb Movements of Sleep (PLMS)
These conditions are when people experience involuntary movements during sleep that waken them. The cause is unknown. It can affect women in the last two trimesters of pregnancy, and has been linked with low iron levels. Sometimes other medical conditions, such as diabetes and Parkinson’s disease, are associated with RLS.
The stages of sleep
The diagram below shows the sequence of sleep stages that a person would typically go through during a night.
(REM = rapid eye movement)
See also Sleep disturbances and Sleep and obstructive sleep apnoea
Original material provided by everybody. Reviewed by Bryn Sparks, sleep physiologist, Sleep Well Clinic, August 2008.
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