Why do we sleep? And what is sleep, anyway?

(Last Updated On: June 2, 2023)

sleepScience Fact: Why do we sleep? And what is sleep, anyway?

In Greek mythology, Hypnos, the god of Sleep, is half-brother to Thanatos, who represents Death. And Shakespeare’s Hamlet fears death as a nightmare from which you cannot escape. Yet sleep only superficially resembles death, and in fact is necessary for life. If the meaning of life includes being healthy, then this kind of deep rest is essential.

A famous 2008 open-source essay by Chiara Cirelli and Giulio Tononi asks “Is Sleep Essential?”. They answer the question by surveying the animal kingdom. They look for animals that don’t sleep at all, that don’t need to rest after staying awake longer than usual, and animals that are not seriously hurt by staying awake. The authors conclude that “sleep is present and strictly regulated in all animal species that have been carefully studied so far.”

Is sleep simply a matter of saving energy? That is not the case. Animals that hibernate put themselves into a state that expends a minimum of energy. And brain wave studies show that animals in hibernation are actually awake. However, they become sleep deprived, and when they rouse to a higher energy state, one of the first things they do is to snooze. Napping must be providing the animal some benefit that makes it worth the extra expenditure of energy compared with hibernation.

Cirelli and Tononi point out that “the most immediate, unavoidable effect of sleep deprivation is cognitive impairment. The brain suffers most from sleep deprivation.” In line with this thinking, the relevant research today accompanies efforts to understand the brain, to which Science just devoted an entire special issue. Studies today start with the smallest parts and go up from there: The detailed chemistry of memorySlice-and-stimulate studies of the brains of trained rats. And whole-brain MRI patterns correlated with words and concepts. 

Some of the most interesting research comes from studying how napping affects memory, learning and creativity. REM (rapid eye movement) sleep accompanies dreaming. However, REM also assists creativity measured by Guilford’s Test. (In this test, subjects try to list a large number of uses for a common household item.)

Conversely, non-REM (dreamless) sleep helps performance on serial memory tasks such as remembering lists of items. Thus different kinds of dozing seem to help different kinds of learning and memory tasks. And of course there are many stories about great insights gained after snoozing, or even after a brief nap.

Science Speculation: So what’s going on here, anyway?

The brain is very different from a stored program computer, certainly in structure and likely also in programming. Nevertheless, the computer can suggest analogies that may also apply to the brain.

You may have opened an application or started a search and seen a message that your computer had not yet finished cataloguing the information. Even when you don’t see the message, this indexing activity is going on in the background, organizing your files so that you can search them. But what if you kept your computer so busy with calculations that it could never find the time to index your files? Quite simply, you would not be able to find things that you wanted!

The analogy we’re looking for is that every waking moment, our senses, especially vision, are taking in a barrage of sensory data. What happens to that information? It’s plausible to believe that it’s kept in some kind of temporary storage until the brain can organize it. The brain might sort through it, keep the few important nuggets, and discard the rest. And sleeping may give the respite that our minds need to do that processing.

This pretty well lines up with what researchers and commentators believe. Here are some expressive quotes:
Wagner et al: “Sleep, by restructuring new memory representations, facilitates extraction of explicit knowledge and insightful behaviour.”
Cirelli and Tononi: “Sleep may be a good time for consolidating and integrating new memories without interference from ongoing activities.”
Greaves: “I also speculate that every night in sleep the mind goes through these physical recollections of the day and disposes of those that are of little consequence and will not help ensure survival of that individual.”
Max, quoting Tononi: “The sleeping brain seems to weed out redundant or unnecessary synapses or connections. So the purpose of sleep may be to help us remember what’s important, by letting us forget what’s not.”

These quotes talk about the information-handling process of sorting through data. That is, discarding what isn’t needed and filing away what is of value. But in the brain, data storage occurs via neurochemistry: changes in synapses, involving protein synthesis and ion transport.

Therefore, it was very interesting when researchers recently reported on these chemical changes. They found that when sleeping, the brain more rapidly eliminates metabolic waste products, increasing the “empty space” between neurons. “Taking out the trash”, the editor summarized it. So in this case neurochemistry seems consistent with our emerging understanding of sleep and memory.

Science is still searching for a full understanding of the brain but we don’t need to wait for that to know what to do. When information overwhelms us, making us confused and going nuts, our best move is to take a nap.

But is that the principal reason we sleep? We can’t yet be sure. As a leading sleep scientist said to sum up fifty years of research, “As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.”

Do you intentionally use sleep as a problem-solving or memory-enhancing tool?

Drawing Credit: nicubunu, on openclipart.org


Why do we sleep? And what is sleep, anyway? — 2 Comments

  1. I am flattered to be quoted on this subject in Art’s blog: such an inelegant word blog but what else? In my world which relies very much on observation I noted that when my kids were in their first year or two, they slept a great deal, and also when doing so their small heads grew almost unnaturally hot so that when strapped to the baby seat in the rear of the car,the window directly adjacent would mist over on cold nights from the heat generated. At that age their small brains must be taking in vast amounts of information about how to move and survive and at a higher rate than they will ever experience later. From that I speculate that such information has to be processed when they sleep into what is useful to survival and what is not, the latter being cast aside which process is happening during sleep. This also accords with my hypothesis that memory operates on a system whereby structures of synapse firings in the brain from the past will tend to resonate with similar structures created in the brain by similar thought structures in the present brought on by whatever current observations or mental conclusions (morphic resonance in the terms of biochemist Rupert Sheldrake).

    However memory also depends on short term sources of stimuli to generate such resonance flows, and my conjecture is that these are physically stored in the brain in the from of very complex molecules similar to if not the same, as DNA, capable of storing vast amounts of information. It is these short term memory molecules that have to be stored during sleep if they are significant enough to survival whilst the rest are cast out. This is why a child’s head is hot and needs so much sleep, and when one is much older as one is from personal experience above the age of 70, then one needs much less sleep. I have learned most of what will help me to survive as well as maybe, and unfortunately there is less need for me to sort out the chaff from the wheat: my best and most efficient years for understanding nature are past, memory deteriorates, at least mine does, and one hands over to the next generation.

    This process of weeding out chaff and then storing useful information physically in the memory is very demanding energy wise even though done unconsciously in sleep (no distractions) and every so often the mind takes a break and slips into neutral when dreaming starts signified by REM, and when the mind will pick up on whatever might have concerned it from recent events or perhaps from older experiences which have now turned into obsessions by too much repetition. I have further thoughts on the nature of dreams, although not yet resolved and that is another subject connected with what happens when synapses fire increasingly randomly as I conjecture they do in the trance state.

    The whole question and problem of the function of sleep seems easy enough to me in terms of how I see things which again is one of the problems of increasing years: lack of flexibility of approach and the sense that one is right and anybody else who sees things very differently is in all probability wrong. When I was younger, it was the other way round: oh dear, what an irony.

    I did not know that during sleep, metabolic waste products are eliminated from the brain, which is exactly what I would expect: very good indeed. I shall have to read more about this. Neither did I know much about memory consolidation and protein synthesis having formed my basic hypothesis on memory in the late 1970s, and having done little further research on it since then. Thanks Art.

  2. After six hours of mental activity (writing fiction), I find an hour’s nap helps clear my mind of the insignificant and distracting, much as a stirred-up mud settles to the bottom of a pool if it’s left alone.