Why Do We Sleep?

photo credit:istockphoto/Kristina Greke
                     photo credit:istockphoto/Kristina Greke

Lets start with the basics, what is it that makes us sleepy? Makes our heads drop off during lectures, texts left unread ’til the morning and afternoon naps such an essential…

It starts with a substance called Adenosine, a by-product of the use of ATP in our bodies day to day chemical reactions. As soon as we wake up in the morning, active processes are taking place and adenosine levels in the forebrain begin to rise. This concentration rises throughout the day and is associated with sleep debt. Experimental evidence has been shown to support this by injecting animals with adenosine and observing an induced sleep. Supplementary to this, using adenosine blockers such as caffeine and other such stimulants inhibits the feeling of drowsiness and need for sleep.

Ever woken up from a sleep and continued to feel half awake for an extended period of time? This is an experience known as sleep inertia. It is again associated with high levels of adenosine, which the body has not had time to sufficiently break down to keep the grogginess at bay. If you’ve ever been abruptly woken up from a nap, during the middle of a deep sleep or after a period of extended sleep deprivation…you’re likely to be familiar with this feeling.

So why do we tuck up in our beds at night and go to sleep? There are many circulating theories, from a variety of varying view points.

Firstly, the evolutionary theory which is based on the need for survival. In summary, it is suggested that if an animal were to sleep during periods at which they are most vulnerable they are likely to be stationary and in an area to avoid any source of danger. This can be shown through animals at the top of the food chain such as bears who have few predators are able to sleep for around 15 hours a day. However those who are lower down in the food chain and are at risk of many predators are able to survive on as little as 4 hours of sleep. However, this theory comes under scrutiny – how is an organism to respond in the case of an emergency?

Next up, the energy allocation theory. During sleep we alternate between rapid eye movement (REM) sleep, and non rapid eye movement sleep (NREM). Endothermic species such as ourselves use REM sleep to concentrate energy into processes involving the central nervous system. In order to do this it directs energy away from unnecessary processes such as metabolism and regulation of body temperature. Whilst asleep, the bodies need for food consumption / calorie intake also declines. Allowing us to store more energy than is used up.

The repair and restoration is probably the most familiar and well known theory. It explains our reasoning for sleep as a chance for the body to repair damage and rejuvenate itself. This has been shown as the bodies rate of cell division and protein synthesis increases and the body has a chance to complete regulatory and house keeping functions. When animals experience prolonged sleep deprivation their immune system begins to fail and they shortly die. Time tucked away snoozing has also recently been shown to be used as time for the brain to remove waste toxins through the glympathic system that have build up throughout the day. It is suggested that the brain has to chose between a conscious state or sleeping in order to ‘clean up’.

There are also theories based on cognitive research. Those such as the information consolidation, and the brain plasticity theories show that sleep is vital for the brain development. Support for such theories has been shown through the increased time asleep during infancy, when are brains are most plastic and absorbing new knowledge daily. As humans we require sleep in order to give our brains time to store information we have learnt throughout the day, without which we are unlikely to recall anything new we have learnt. Sleep is also a necessity in order to learn and perform tasks, if we are deprived both of these normal functions become much more impaired.

As you can see, there are many sources of evidence suggesting varying reasons for why we need sleep. To put it simply, there is no clear cut reason or theory that can outcast the others. It is likely to a mixture of them all!

If you have science questions that you’d like to know the answer to in a future post please tweet me and let me know! @basicallylorna

Lorna Milne

19 year old, Biomedical Science undergraduate at the University of Sheffield.