It would be surprising if there were a single function for sleep, but there are often articles implying that the mystery is solved and THE reason for sleep has been found. Recently there was one in the New Scientist which prompted my post (https://www.newscientist.com/article/2096921-mystery-of-what-sleep-does-to-our-brains-may-finally-be-solved/).
We can look logically at reasons why we sleep. For any biological behaviour or process, there can be a spectrum of causes. At the one end of the spectrum are the ultimate causes – the evolutionary reason, the function being carried out, and the ‘why’. At the other end are the proximate causes – the individual immediate cause, the trigger, and the ‘how’.
The thing that seems the most distance and universal ultimate cause is probably that it is evolutionarily difficult to be well fitted to two dissimilar niches at the same time. There are animals that are active in the day and adapted to that; they hid and conserve their energy at night because they are not adapted to night. Or an animal can be adapted to night and hid in the day. This idea applies to animals that hibernate through a cold season every year to which they are not well adapted. There are animals that go dormant in dry seasons and are active when it is wet. It would be a good bet that all other functions of sleep are built on this mechanism of being inactive during recurring periods. Sleep is widespread amongst animals.
If there is an inactive time, that is the time to do all the things that cannot be done easily when active. Growth and repair would be immensely easier during rest. Imagine growing new muscle cells in a walking leg – much easier to wait until the leg is not moving. Growth and repair is best done on all the organs when they are just ticking over at most. This would include any maintenance needed in the brain.
These ultimate causes produce, during evolution, a proximate mechanism: an oscillating function that produces drowsiness and than sleep and followed by awakening and activity. Hormones and signals can work this rhythm without triggers, but do use light and darkness to get the timing right. The mechanism also affects the working of the body to make it suitable for growth and repair – the temperature, heart and breathing rates and many hidden levels have different sleep and wake settings. This sort of mechanism causes sleep but it is the ‘how’ of sleep not the ‘why’.
The brain is a complex organ with many functions. It is not a simple concept to say that the brain is inactive, resting and recuperating. There may be many processes in the brain taking advantage of sleep; there are a number of different types of sleep which obviously have different functions.
One function that has been investigated is waste removal. During parts of sleep some of the brain cells become physically smaller and this allow the movement of liquid through the brain, clearing out waste. It probably would interfere with the working of the brain to have cells temporarily lose volume – better to do this during sleep.
Another function that sleep houses is REM phase/dreaming. Dreaming is not completely understood but it is clearly needed for the successful integration of new memories into the web of established memories in the cortex. In this process the memories seem to be partially activated and re-experienced in combination with previous memories. For this process to be safe (without sleep walking or worse) the brain disconnects the possibility of skeletal muscle movement. This paralysis could only be acceptable in the safe inactive state of sleeping.
Another function that is known but not completely understood is the resetting of the brain’s level of activity. This is the one outlined in the NewScientist article. Consider that during the day, there has been continuous firing of neurons as a result of continuous signaling through the synapses of the brain. The synapses that are not used do not lose any strength while the ones that are used increase in strength. We end the day with a very excitable brain. During sleep this is brought down to a lower level to start the next day. All the synapses lose strength so that the new difference between strong ones and weaker ones remains but the overall strength is lower. As a photographic metaphor, it is like lowering the ‘brightness’ while maintaining the ‘contrast’ on a washed out over exposed photo. If this is not done, due to sleep deprivation, then the brain finds it more and more difficult to function. It seems that sleep alternates between strengthening particular synapses and weakening all synapses.
Of course, this is probably only the tip of the iceberg for finding sleep functions in the brain. I would not be surprised at a number of others being identified. There may even be processes that apply to insects but not to humans, as insects can sleep too. That is why an article that implies that there is a single function, THE reason for sleep, and that single reason has finally been found, is annoying. All the causes of, reasons for, functions of sleep are important and unlikely to be all found or understood.