A: It's evolution, baby
Deep non-REM sleep appears to affect how well we commit a new task to memory
Sleep specialists like to divide sleep into one of two states: rapid eye movement sleep (REM) and non-rapid eye movement sleep (non-REM). Since the discovery of REM and its tight link to dreaming in 1953, there has been a lot of research focused on this paradoxical wake-like state. But as we experience much more non-REM than REM during the night, non-REM or deep sleep might be the more important of the two states.
It’s likely there are many functions of non-REM. It could simply be a means of energy saving, reducing metabolic output during naturally selected hours of inactivity. Non-REM could involve some kind of neurological reset, allowing the neurons to replenish neurotransmitters. Perhaps the downtime is used to clear metabolites. There may be synaptic strengthening. There may be synaptic pruning. Both of these could enhance brain performance. It’s possible that non-REM performs all of these functions, and more besides.
There is an interesting experimental study on non-REM in Nature Communications this week. Researchers in Switzerland gave volunteers the task of learning a specific sequence of six finger taps, rather like asking them to learn to play a six-note ditty on the piano with the fingers of one hand but without the sound. After learning, as the subjects slept, some received a pulse of “transcranial magnetic stimulation” (TMS) directed at their motor cortex and timed to coincide with the deep, non-REM state. Although these experimental individuals didn’t notice they’d been targeted during the night and reported sleeping just as well as controls, they did less well at the six-finger memory task when tested the following day.
It’s a small study, but it does suggest that targeted TMS could be a useful experimental tool in efforts to figure out what is going on in the different stages of sleep. There is a therapeutic flip-side too. TMS is increasingly deployed for the treatment of depression and, appropriately delivered, might be able to enhance rather than disrupt non-REM sleep, thereby improving learning and memory (see this study, for instance).
In the last sentence, the words “appropriately delivered” are fairly important because I can see the growth of a poorly evidenced TMS industry, with desperate sleep-disordered people donning magnets at night and synching them with beautiful apps that promise stimulation at just the right frequency at just the right moment.
This would be about as silly as trying to run before walking. Sleep science is still in its infancy and it would seem wise to get a better handle on what is happening in our brains when we sleep before we begin self-medicating with magnets.