Sleep is critical to health, both mental and physical, but scientists have been working for ages to try and elucidate why. It's known that when deprived of sleep short-term, people are groggy, irritable, and perform more poorly on every cognitive assessment there is; long-term sleep deprivation causes hallucinations, paranoia, and (ultimately) death. Further, sleep is not the same as simple bodily relaxation; in some phases of sleep, the brain is as active as it is during wakefulness.
[image courtesy of photograph Evgeniy Isaev and the Wikimedia Commons]
This doesn't, however, explain sleep's function in other animals, as few fluffy bunnies are known to take sociology classes. But a study published last week in Nature: Communications has shown that sleep definitely influences memory -- even to the extent of fostering the formation of new memories while we're asleep.
The paper, "Formation and Suppression of Acoustic Memories During Human Sleep," by Thomas Andrillon, Daniel Pressnitzer, Damien Léger, and Sid Kouider of the École Normale Supérieure/PSL Research University of Paris, France, found that people form memories during REM (rapid eye movement) sleep -- but stimuli given during deep non-REM sleep actually suppress memory. The authors write:
Sleep and memory are deeply related, but the nature of the neuroplastic processes induced by sleep remains unclear. Here, we report that memory traces can be both formed or suppressed during sleep, depending on sleep phase. We played samples of acoustic noise to sleeping human listeners. Repeated exposure to a novel noise during Rapid Eye Movements (REM) or light non-REM (NREM) sleep leads to improvements in behavioral performance upon awakening. Strikingly, the same exposure during deep NREM sleep leads to impaired performance upon awakening. Electroencephalographic markers of learning extracted during sleep confirm a dissociation between sleep facilitating memory formation (light NREM and REM sleep) and sleep suppressing learning (deep NREM sleep). We can trace these neural changes back to transient sleep events, such as spindles for memory facilitation and slow waves for suppression. Thus, highly selective memory processes are active during human sleep, with intertwined episodes of facilitative and suppressive plasticity.It's important to add, however, that experiments involving attempts to learn something complex -- such as a foreign language -- while you're asleep have all been abject failures. The (alleged) phenomenon, called hypnopaedia, was all the rage back in the 1920s and 1930s, when people not only used it to try to learn, they used it to modify how they were perceived by others. One such program, developed by hypnopaedia's main proponent, New York psychologist A. B. Saliger, was designed to help people get laid. It involved listening while asleep to the following message played over and over: "I desire a mate. I radiate love... My conversation is interesting. My company is delightful. I have a strong sex appeal."
Unfortunately, all that this accomplished was making the test subjects wake up even hornier than they were before.
Study author Thomas Andrillon is cautious about applications of his work to the practical world. "The sleeping brain is including a lot of information that is happening outside, and processing it to quite an impressive degree of complexity," Andrillon said. But as far as using this research to facilitate the development of methods to influence memory in a bigger way, he added, "We are in the big unknown... Keep in mind that sleep is not just about memory. Trying to hijack the recommended seven-plus hours of sleep could disrupt normal brain function."
Which I can certainly attest to, as I rarely if ever get seven hours of uninterrupted sleep, and I don't think I've ever had the word "normal" applied to my behavior. I have to admit that the Andrillon et al. research is fascinating, however, not least because it gives us another piece of the puzzle of why all higher animals sleep.
Now you'll have to excuse me, because I got about three hours of sleep last night, and I think I'm gonna go take a nap.