In recent years, sleep has been less of an enigma than it was in the 70s and 80s. Memory consolidation is now well known as the prime reason why we actually need sleep, but there is still a lot of controversy regarding the exact mechanism by which sleep alters the learning characteristics of the brain. While the effects of long-term sleep deprivation are much better studied and characterized, the role of napping as opposed to deep slumber has been the subject of a recent study.

Researchers at the University College of Berkley, California, studied in particular the role of the traditionally neglected non–REM phase of sleep in the learning process on a group of 44 volunteers, by subjecting them to rigorous tasks aimed at the hippocampus in particular — memorization. During the non-REM phase of sleep (where there is no rapid eye movement or REM), sharp spikes of electrical activity called sleep spindles were recorded from hippocampal region. Normally, these spikes occur about a 1000 times per night, and are thought to be associated with the process analogous to scrubbing the hippocampus free of short-term memory traces, and helping further short-term memory at accumulation once we wake up. In this study, half of the subjects were allowed to have a 90 minutes nap in between two heavy learning sessions in the afternoon and in the evening. These sleep spindles were noted in the above group, and they typically demonstrated better learning in the evening session, compared to the other half who were not allowed to sleep.

One of the important implications of the study is that non-REM sleep serves an important purpose than commonly thought — recharging the brain for learning. Concomitantly performed electroencephalogram (EEG) studies mapping the brainwaves of the participants showed that there was a distinct correlation between the amount of sleep spindles and the quality of learning soon afterwards. These spikes where seen selectively in the hippocampus, also looping to the prefrontal cortex, the two parts of the brain that are thought to be the key areas involved in learning. Walker, the lead researcher of this study published in a recent edition of Current Biology, stated that sleep selectively restores critical learning functions of the brain. In their opinion, non-REM sleep deprivation typically seen in the older population may account for the reduced memorization capacity during learning. Perhaps it also points out that in chronically sleep deprived individuals, learning performance is justifiably diminished.

But what is the exact mechanism at the cellular level by which sleep affects our learning?

This remains a controversial area within neurology at present and the debated theory of synaptic pruning is a leading candidate with some recent experimental evidence. This theory was first proposed nearly seven years ago by Cirelli and Tononi, neurobiologists based at the University of Wisconsin-Madison, which is essentially based on the idea that when awake, the synapses within our brain grow stronger and proliferate leading to a form of neurological saturation. During sleep there is a massive downscaling of such synaptic connections, thus freeing up nerve cell resources for further learning once we are awake. Despite some supporting evidence in fruit flies and fishes, this theory needs further verification before it is accepted as a general model of how learning occurs in humans. Nevertheless it can explain some of the features of the non-REM sleep enhancement of learning ability seen in the subjects of the UC Berkley study published this earlier this year.

Does sleep therefore help in pruning and unentangling the nerve endings in our brain?

Perhaps time will tell.

References

Mander, B., Santhanam, S., Saletin, J., & Walker, M. (2011). Wake deterioration and sleep restoration of human learning Current Biology, 21 (5) DOI: 10.1016/j.cub.2011.01.019

Tononi, G. (2003). Sleep and synaptic homeostasis: a hypothesis Brain Research Bulletin, 62 (2), 143-150 DOI: 10.1016/j.brainresbull.2003.09.004