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The science of sleep

We spend about a third of our life asleep but have you ever thought about why you sleep? And how much you should be getting?

Here we explain the science behind sleep to give you a better understanding of this most fundamental of needs.

Slippers by a bed


  • Why do I need to sleep? Why do I need to sleep?

    Sleep is a bit of a mystery. We still don’t fully understand why we need to sleep and this has long been the subject of debate. There are several theories but the two that seem to stick are that sleep:

    • allows your body to replenish energy stores and repair itself
    • gives you the chance to save energy

    Sleep may also aid learning and give our bodies time to organise all the memories of the hundreds of events we experience every day.

    Did you know...?

    We know much more about the effects of not getting enough sleep. It’s associated with less concentration and feeling tired and irritable, as well as depression. This suggests sleep is important for our brains to work properly.

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  • How much sleep do I need? How much sleep do I need?

    The amount of sleep we need changes as we get older, and it’s also a personal thing – some of us need more than others. As long as you’re feeling refreshed and alert the next day, you’re probably getting enough.

    Although there is no magic number of hours, here’s a general guide for how much sleep you need.

    Stage of life Hours
    Babies 17
    Toddlers 12 to 14
    Children 9 to 12
    Teenagers 9
    Adults 8
    Older adults 8

    Did you know...?

    It’s common for teenagers to stay up late and complain about feeling too tired to get up for school but there may be some biological theory behind this. Some teenagers get a condition called delayed sleep phase disorder in which their body clock doesn’t work properly. They have a natural inclination to go to bed late and wake up later than is considered normal.

  • Why do I fall asleep? Why do I fall asleep?

    You might not have given much thought to what causes you to fall asleep but there are various chemical processes in your body at work. Some of these keep track of how long you’ve been awake and when you need to sleep. Others determine what time of day is best to sleep. These processes interact with each other and with your environment to make you feel sleepy at about the same time every day.

    You also have an internal body clock, which is governed by daylight. Cells in your eyes detect this and stimulate your body to produce a light-sensitive chemical called melanopsin which makes you feel awake in the daytime. As it gets darker, less of this chemical is produced and instead your body produces something called melatonin which makes you feel sleepy.

    Did you know...?

    Your internal body clock isn’t fully developed when you’re first born. This goes some way to explaining the sleepless nights many new parents know only too well. It develops it in the first few months of your life and then you gradually adjust to sleeping more at night than during the day.

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  • What happens when I go to sleep? What happens when I go to sleep?

    You pass through different phases of sleep in regular cycles during the night. The two main phases are:

    • rapid eye movement (REM) sleep
    • non-rapid eye movement (non-REM) sleep

    You move through stages of non-REM sleep then into REM sleep and back again. Each full cycle takes about one to two hours.

    The sleep cycle

    Click on the image to enlarge it.

    The sleep cycle

    Did you know...?

    Active dreams and nightmares can start to be a real problem for toddlers. It’s a good idea to take some practical steps to try to prevent your child having nightmares. So, for example, it might sound obvious but make sure your toddler doesn’t watch anything frightening on television before they go to bed.

  • Why can't I sleep? Why can't I sleep?

    It’s normal to wake up during the night every couple of hours for a minute or so. You won’t usually be aware of these brief interruptions to your sleep. But if you’re disturbed by something when you wake up – maybe your partner snoring – it may make it harder to go back to sleep. These breaks in your sleep may also become more noticeable if you’re feeling anxious about something.

    If this becomes a regular problem and you start to get insomnia, you’ll feel tired during the day. For tips on how to tackle this, see our article How to get a good night’s sleep.

    Did you know...?

    We get less deep sleep as we get older so can wake more easily after the first three to four hours of sleep. Other health issues may also have an impact on the amount and quality of sleep you get. For example, if you’re in pain from osteoarthritis.

  • Resources Resources

    Headspace This tool describes itself as gym membership for your mind using meditation and mindfulness techniques. You can start off with free 10-day introduction to meditation and then choose to subscribe for access to more exercises covering a range of topics. You can use it on your phone or computer, depending on what suits you best.
    Mental Health Foundation The Mental Health Foundation is a charity that carries out research and offers information about many areas of mental health. If you have problems with your sleep, this page is for you. It has details of some reasons that are behind having trouble sleeping and suggestions of things you can try to help yourself sleep better.
    Mind The charity Mind has information to support people with a mental health condition and those who care for them. Their sleep content has tips on practical things you can do to help yourself if you’re not sleeping well.


    • Khan S, Heussler H, McGuire T, et al. Melatonin for non-respiratory sleep disorders in typically developing children (protocol). Cochrane Database of Systematic Reviews 2011, Issue 5. doi:10.1002/14651858.CD009141
    • Normal sleep, sleep physiology, and sleep deprivation. Medscape., published 22 October 2013
    • Sleeping well. Royal College of Psychiatrists., published July 2014
    • Insomnia. BMJ Best Practice., published 21 January 2015
    • Sleep problems in childhood and adolescence: for parents, carers and anyone who works with young people. Royal College of Psychiatrists., published January 2012
    • Gradisar M, Crowley SJ. Delayed sleep phase disorder in youth. Curr Opin Psychiatry 2013; 26(6):580–85. doi:10.1097/YCO.0b013e328365a1d4
    • Pineal gland and circadian rhythms. PatientPlus., reviewed 6 September 2013
    • Iwata O, Okamura H, Saitsu H, et al. Diurnal cortisol changes in newborn infants suggesting entrainment of peripheral circadian clock in utero and at birth. J Clin Endocrinol Metab 2013; 98(1):E25–32. doi:10.1210/jc.2012-2750
    • Clinical practice guideline on sleep disorders in childhood and adolescence in primary care. Ministry of Health Social Services and Equality., published 2011
    • Geriatric sleep disorder. Medscape., published 23 September 2014
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    Reviewed by Rachael Mayfield-Blake, Bupa Health Content Team, August 2015.

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