What happens while we are sleeping and why do we need to sleep?
What is sleep?
We all know we need it and suffer when we don't get enough, but what is sleep and why do we need it? Some people have likened sleep to a complete shut down, like turning off a computer. However, sleep is more like a prolonged "re-boot" where files are put into order. It is difficult to describe exactly what sleep is, but it is an active process and is essential for growth, repair, memory, and ultimately, life.
What happens during sleep?Most adults function best on 7 or 8 hours sleep each night, although some people need more and some less. As long as you feel rested and awake when you need to be, you are probably getting enough sleep1.
Sleep is a biological need for all animals. It helps us to rest and recuperate physically and psychologically, and to grow and regenerate body cells. Some scientists believe that sleep plays a role in developing memories2.
The biological clockOur levels of sleepiness and wakefulness are partly regulated by our biological clock, which increases certain chemicals in our body to make us feel more or less awake, and external stimulus factors such as night and day. Difficulties in sleep arise when our body clock is disrupted and forced to work in an inappropriate way, for example, during shift work or when suffering from jet lag.
What are sleep cycles?Sleep has many layers and stages but the most important division is between REM (Rapid eye movement) and NREM (Non-rapid eye movement) sleep1. A sleep cycle is composed of both REM sleep and NREM sleep.
REM sleep - REM stands for rapid eye movement, because your eyes periodically move rapidly during this sleep stage, where brain activity is high. It is also when you dream. Your muscles become temporarily paralyzed during this kind of sleep to prevent you from acting out your dreams.
NREM sleep - During NREM (non-rapid eye movement) sleep, brain activity is much slower. However, you are able to move more naturally, and it is during this stage that sleepwalking and talking take place. Research suggests that rest and restoration also take place during this time.
NREM sleep is divided into four stages. During the night you pass through these different stages from light sleep in stage 1 and 2, to deep, restorative, slow-wave sleep in stages 3 and 4. The amount you spend in each stage of sleep varies depending on your sleep need and usual sleep pattern. You move up and down in these four stages during the night and find it much easier to wake up if disturbed during phase 1 than during phase 4.
How do sleep cycles work?Each cycle of NREM and REM lasts approximately 90 minutes. At the end of each cycle you wake up, but often without realizing it, because your brain encourages you to go back to sleep immediately to start a new cycle. Most people are not aware that they woke up during the night, however some people do remember this. This isn't a problem, unless you wake up and stay awake. In total, adults spend about 20 percent of the night in REM sleep, while babies spend about half their time in REM sleep.
Test - how long is your sleep cycle?You can find out how long your own sleep cycle is with an easy experiment. Next time you are forced to stay awake past your natural sleeping time, for example, at a social occasion or if you have to work very late, and you are feeling so tired that you could fall asleep, record the time. If you stay awake through this time, in about 15-20 minutes you will start to perk up again and feel more alert. However, the feelings of extreme tiredness will return - the time between your first strong feelings of tiredness and your next wave of tiredness is your sleep cycle. Most people have a cycle of about 90 minutes.
- Dement W. The promise of sleep. Macmillan. 1999
- Idzikowski C. Insomnia. Newleaf. 1999
- National Institute of Neurological Disorders and Stroke (USA). Brain basics: understanding sleep. 2000; www.ninds.nih.gov
- Sleep Home Pages. Basics of sleep behavior. Part A. What is sleep-1998; www.sleephomepages.org/sleepsyllabus
- Pritzker School of Sleep Medicine, University of Chicago.