When you go to bed and close your eyes you should begin to drift off within 20 minutes or so. Passing rapidly through sleep stages 1, 2 and 3, you reach stage 4, deep sleep. Then comes REM (Rapid Eye Movement) sleep, when your eyes begin to flicker and you experience your most vivid dreams. After REM you begin to wake up a bit, before going back down into deep sleep. This cycle is repeated every 90 minutes or so throughout the night.
During deep sleep our brains work hard, moving memories from short-term into long-term storage, allowing us more short-term memory space for the next day. If you don’t get adequate deep sleep then these memories may be lost.
You might think: “I’ll cut back during the week then make up for it at the weekend.” Unfortunately, it doesn’t work like that, because memories need to be consolidated within 24 hours of being formed.
Since deep sleep is so important for consolidating memories, it’s a good idea if you are revising or taking an exam this summer to make sure you’re getting enough sleep. In one study, people who failed to do so did 40 per cent worse than their contemporaries.
Flush out the toxins
Another important thing that seems to happen in deep sleep is that toxins get washed out of the brain. What’s been found, at least in mice, is that the space between brain cells can increase during sleep, allowing the brain to flush out toxins that build up during the waking hours.
A build-up of toxins, caused by lack of sleep, may in turn make you more vulnerable to diseases like Alzheimer’s. Scientists have recently shown that the sticky amyloid plaques that are associated with Alzheimer’s disease develop more rapidly in the brains of mice if you deprive them of sleep.
Stay calm — and sleep on
When you are in REM sleep, stress-related chemicals in the brain, like noradrenaline, are switched off. It’s the only time, day or night, that this happens. It allows us to remain calm while our brains reprocess all the experiences of the day, helping us come to terms with particularly emotional events.
We get more REM sleep in the last half of the night. Which means that if you are woken unexpectedly, your brain may not have dealt with all your emotions – which could leave you stressed and anxious.