We spend a third of our lives asleep, so sleep is important to us even if many do not know why. Generally speaking, sleep duration is determined by how vulnerable we are as a species. Zebra sleep two hours a night, because they may be eaten by lions otherwise (!) The Giant Sloth sleeps 20 hours a day – and it does so because it can. It lives in the highest jungle canopies, it is the largest animal up there with no natural predators, and all of its food is within easy reach.

Human beings are predominately visual creatures, with roughly a third of our brain capacity involved. We see in wonderful colour but the consequence of that is poor night vision. For us to be active at night is to stumble over cliffs in the dark and to be at the mercy of predators, wasting precious energy to no real effect. So we sleep on average eight hours a night. This used to vary with season (going to bed at sunset, getting up at sunrise) but our lives are now illuminated by artificial light which throw our natural rhythms out of alignment.

Sleep, in itself, is easy. We find a cold, dark and quiet spot, we lie down and lay still, and then sleep takes us. However, if daylight is present, or if primitive components of our brain are tricked into thinking it is daylight, it's not so easy. There is a part of the brain called the Suprachiasmatic Nucleus (or SCN). It is part of the hypothalamus, and we know it best as our 'body clock'. It receives information from the visual system, specifically from neurons in the retina which are sensitive to blue light. TVs throw out blue light. So do PC monitors. Watching TV or playing computer games before bed can interfere in our ability to tire, get to sleep, and stay asleep.

Falling asleep is only part of it. There are a number of stages to sleep, and we need to be asleep for some time, before we descend to slow-wave-sleep (SWS). Here, activity is limited mainly to the workings of the body – important functions such as heart-beat, breathing, etc. – and this is the time when the body is best able to repair and restore the damage of the previous day – micro-fractures in the bones of the ankles from climbing stairs, or damage to the skin from the action of the sun – so failure to get a good night's sleep results in less effective repair.  As we age, it becomes harder to reach and maintain SWS. So over the years, damage accumulates and we begin to visibly age.

In the second half of the night, we spend less time in SWS, and more time in Rapid Eye Movement (or REM) Sleep. Here, the brain is going through the actions of the previous day and is trying to make sense of and store facts and experiences. Apart from the eyes, the body is rendered paralysed so that we do not act out our dreams and injure ourselves. (When this system is not working well, we see conditions such as sleepwalking etc.). If someone is woken from SWS, they will be sluggish and disorientated as the brain needs to switch itself back on. But if woken from REM sleep, the brain is fully active and people can respond more quickly. Indeed, people are surprisingly aware of what is going on around them, even if they will not remember it later. For example, if water is dripped on the face of someone dreaming and then they are woken up, they will often report that they were dreaming of being on a ship or that they were swimming.  Sleep, ultimately, is about flip-flopping between SWS and REM. It restores us, and it enables us to better remember the events of our lives. As such, it is time well-spent.

Dr James Jackson is a Senior Lecturer, Psychologist, and Tinnitus Researcher at Leeds Trinity University. Dr Jackson's primary research interest is hearing loss and tinnitus, and he speaks at charity conferences all over the UK.