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.
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 in search of 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. They 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. Sleep 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 is closely tied with 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 memory; slice-and-stimulate studies of the brains of trained rats; whole-brain MRI patterns correlated with words and concepts.
Some of the most interesting research comes from studying how sleep affects memory, learning and creativity. REM (rapid eye movement) sleep, which is noteworthy for its association with dreaming, seems associated with creativity as measured by Guilford’s Test, in which 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 sleep 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, only to see 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 they can be searched. 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 sort through it, keep the few important nuggets, and discard the rest. And sleep 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, discarding what isn’t needed and filing away what is of value. But in the brain, data is represented by neurochemistry: changes in synapses, involving protein synthesis and ion transport. Therefore, it was very interesting when researchers recently reported that when sleeping, metabolic waste products are more rapidly eliminated from the brain, 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 we are overwhelmed with information, 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