Dreaming regenerates your mind

As far as the mind is concerned, the most critical phases of sleep are bursts of REM (rapid eye movement) sleep, when dreams occur most frequently. These tend to start after an hour or so of sleep and last for about 30 minutes, occurring on average five to seven times a night. The longer you sleep, the longer the REM phases last, which is why you need at least seven hours a night.

When people are deprived of REM sleep, they don't feel fully rested on waking and are more likely to get depressed. When they do gel a chance to sleep they have longer periods of REM sleep. All of this suggests that our minds need to have this time while we're asleep to process what's been happening in our lives. It is generally thought that dreams are important for mental and emotional health.

Better memory

During REM sleep the brain makes all sorts of neurotransmitters, from noradrenalin, which keeps you stimulated, to serotonin, which keeps you happy and improves your memory Dr Robert Stickgold at Harvard Medical School has run sleep experiments, and the results suggest that we need sleep to remember what we've learnt during the day. He found that the ability to perform a recently learnt task improves after six to eight hours sleep. Without sleep, and without dreaming, you don't retain what you've learnt and all your memories from the day stay backed up.

Improved mood

Getting enough REM sleep also improves your mood - don't forget the brain makes serotonin, the happy neurotransmitter, during sleep. Both alcohol and many anti-depressants suppress REM sleep. That's probably why you wake up grumpy if you go to sleep under the influence of alcohol. So, before you reach for the antidepressants, make sure you are getting enough good-quality sleep.

Physical rejuvenation

Within half an hour of falling asleep your body begins a miraculous process of rejuvenation. All through the night, and especially during deep sleep and REM phases, the brain produces higher levels of human growth hormone. This hormone helps with the repair and replacement of both tissue and bone. During the daytime, growth hormone levels are lower. When you're stressed, the subsequently high levels of the stress hormone Cortisol further suppress growth hormone, diverting energy away from repair into coping with the energy demands of a stressful situation.

Normally, levels of the stress hormone Cortisol are low when you sleep. But at times of chronic stress, Cortisol levels may not drop sufficiently while you sleep. This further suppresses tissue repair, effectively speeding up the ageing process. So it's best to go to sleep unstressed, and a good idea to avoid late-night horror movies or the late-night news. Instead, do something relaxing and pleasant before going to sleep.

Live longer

The effects of sleep are far-reaching on your health. Both too little and too much are associated with an earlier death. Statistically, the amount of sleep that correlates with the longest lifespan is between five and nine hours a night. Particularly as we age, there is a higher correlation between few (less than five) and great (more than nine) hours of sleep and increased mortality. Seven hours sleep is linked to the lowest death rate. But just as important is the quality of sleep - many people, as they age, have more fragmented and light sleep and don't get enough slow wave or REM sleep.

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