What is the function of sleep? Researchers have shown that memories are replayed and even enhanced as we sleep. In other words, sleeps aids memory consolidation. But how does that work?
One explanation is based on the oscillations in electrical activity that occur naturally during different sleep phases throughout the night. Oscillations are fastest during rapid eye movement (REM) sleep, and slowest in phases of deep sleep. Previous research has shown that if people learn something before sleeping, their brains show more slow-wave activity when they are asleep. This suggests a correlation between slow oscillations and memory enhancement.
Born and his colleagues have shown that slow oscillations are indeed the cause of memory boost during sleep. The researchers had volunteers memorize and recall words before going to sleep. Electrodes were then positioned on each volunteers’ scalp and, once the volunteers were in a phase of slow-wave sleep, the researchers induced a weak electrical current. This current boosted slow oscillations. The volunteers then slept undisturbed until morning, when they were again asked to recall the words learned before going to sleep.
Volunteers who received brain stimulation remembered more words after sleep than before sleep. The volunteers who did not receive brain stimulation remembered the same number of words before and after sleep. This showed that boosting slow waves resulted in a memory boost. Interestingly, there was no
memory boost when the researchers applied the stimulation at the end of
the night, rather than the beginning.
Of course, this method cannot be easily used at home to improve
memory for tomorrow meeting! We are still a long way to understand
how sleep helps improve memory but one can dream on…. One day
doctors may be able to boost the brain’s waves to compensate for the
reduction in slow-wave sleep that comes with aging.
References: Marshall, L., Helgadóttir, H., Mölle, M., & Born, J. (2006). Boosting slow oscillations during sleep potentiates memory. Nature, 444, 610-613.