We spend almost 30 years of our lives asleep, however, we still have limited understanding about the function of sleep. Scientists and philosophers have been trying to understand why animals need sleep for more than 2000 years. In his book “On Sleep and Sleeplessness”, Aristotle wrote:
“With regard to sleep and waking, we must consider what they are: whether they are peculiar to soul or to body, or common to both; and if common, to what part of soul or body they appertain: further, from what cause it arises that they are attributes of animals, and whether all animals share in them both, or some partake of the one only, others of the other only, or some partake of neither and some of both”
In 17th century scientists thought that sleep comes from our bellies. You know that sleepy feeling you get after having a lunch? Well, scientists thought that when food was digested warm gasses spread through our bodies and make us want to sleep. We now know that although it might be a good excuse for falling asleep during a lecture, that is a complete misunderstanding and it has been clearly shown that the sleep originates in our brain.
The brain is made up of billions of cells, called neurons that actively communicate with each other through electrical and chemical signals. When we are awake these signals are translated into particular actions: walking, running, singing, dancing etc. For a long time it was considered that during sleep these signals die down and the brain enters a kind or state of irresponsiveness, which allows it and the body to recover from a hard day’s work. However, once the development of electroencephalogram technique allowed us to measure electrical signals made in the brains of sleeping and awake people, it was noted that while the patterns of signalling differ in the two states, the brain is still highly active during sleep.
Two types of signals were later described to be made in the ‘sleeping’ brain; these are called REM and Non-REM. REM signals, which stand for Rapid Eye Movement signals, are produced in the brain when the most the most vivid dreams occur. REM is also associated with release of molecules that inhibit skeletal muscle movement. Interestingly, these molecules do not affect muscles that control the movement of the eyeballs, which is why the eyes under the eyelids are still able to move during this stage and why it is called the rapid eye movement stage.
The other signals made in the brain during sleep are creatively called Non-REM signals or Non-Rapid Eye Movement signals. They are made during a much more ‘relaxed’ stage of sleep when our hart rate and breathing slows down, fewer or no dreams occur and we slowly enter a very deep sleep. Non-REM signals can be further divided into 3 or 4 stages beginning with transitioning into sleep and finishing with so called slow-wave-sleep (SWS), the length of which correlates with the amount of “sleep debt” (lack of sleep). SWS is also a stage during which sleep-walking and sleep-talking occurs
REM and Non-REM stages cycle one another during the sleeping period, each lasts for around 90 minutes and the amount of time spent in REM tends to increase towards the end of sleeping period. After around 4-6 cycles ‘ascending arousal system’ takes over the brain and drives wakefulness through release of a number of different neurotransmitter. It should be noted that timing of REM and Non-REM changes throughout our lifetimes, for example, babies spend large amount of time in REM stage which is thought to be required to process the vast amount of incoming information at early stages of life. By contrasts, in adults REM and late stage NREM sleep is greatly reduced which is why they are easily awakened.
We spend on average 8-9h asleep every day and it is not only humans who like to have a healthy power nap; pigeons, elephants, gorillas and all sorts of other animals need sleep. As a matter of fact the koala would win the sleeping championships because it spends 22h asleep every day! So clearly sleep is vital for many animals but why? The easy answer would be that our brain and body needs time to recover after the working day, but as it is clear that the brain remains quite active during sleep as well so clearly there is more to it than that.
Additional evidence has now suggested that sleep is required to remove toxins and repair damaged cells in the brain. Body’s metabolism, the chemical processes that happen in living organisms, generate many byproducts that are can be toxic to the cells. Sleep potentially removes these damaging byproducts. In fact, small animals which have higher metabolic rates (and potentially accumulate more of this metabolic waste) also tend to require more sleep compared to large animals, e.g. mouse sleeps around 12h a day whereas an elephant only needs 3h. A study in Science magazine has showed that sleep can dramatically increase the space between the tissues in the body, which allows greater exchange between cerebrospinal and interstitial fluids to occur. In turn, the this exchange clears the brain of neurotoxins such as beta-amyloid, a known major contributor to Alzheimer’s disease.
Sleep is also essential for learning and converting experiences into memories. Memories are produced in the brain through making new active connections between neurons. When we sleep these memory connections are strengthened by replaying the events that happened during the day. The replay of previous events can also help us learn new things. Experiments have shown that e.g. having a nap between learning to type a random sequence of numbers and repeating the task significantly improved a person’s ability to type the sequence faster and more accurately.
Many perhaps have heard their parents say that you need to sleep in order to grow, which probably derives from the fact that growth hormone levels increase during sleep and it has been linked to increased growth babies. The balance of other hormones is also affected by sleep. For example, ratios of hormones leptin and ghrelin, which control our hunger, change. During sleep deprivation amount of ghrelin increases leading to increased craving for food. Indeed, people who sleep less than 6h have 3-5 fold increase in the risk for diabetes compared to longer sleepers.
National Sleep Foundation recommends 7-8h of sleep for adults but it is important to note that each individual will have different requirements for the amount sleep depending on their age, exact pattern of circadian rhythms and many other aspects. I quite like this brainpickings infographic about the habits of sleep of different writers.
A lot of information for this article has been taken from Lockley, Steven W. “Sleep: A Very Short Introduction (Very Short Introductions)”.