In fact, it is at this time that it is busy storing things you learned that day.
These are the things you'll need in your memory tomorrow, next week, or next month.
Now, for the first time, UNC School of Medicine researchers are using transcranial alternating current stimulation (tACS) to target a specific kind of brain activity during sleep to strengthen memory in healthy people.
Researchers have for years been able to record electrical brain activity that alternates during sleep.
These waves are called sleep spindles, and scientists have suspected their involvement in cataloging and storing memories as people nap.
"We didn't know if sleep spindles enable or even cause memories to be stored and consolidated," said senior author Flavio Frohlich, PhD, assistant professor of psychiatry and member of the UNC Neuroscience Centre.
"They could've been merely byproducts of other brain processes that enabled what we learn to be stored as a memory.
“But our study shows that, indeed, the spindles are crucial for the process of creating memories we need for every-day life. And we can target them to enhance memory."
This is the first time a research group has selectively targeted sleep spindles without also increasing other natural electrical brain activity during sleep.
This has never been done with tDCS (transcranial direct current stimulation) the much more popular cousin of tACS in which a constant stream of weak electrical current is applied to the scalp.
Frohlich's research involved 16 males who underwent a screening night of sleep before completing two nights of sleep for the study.
Before sleeping each night, they performed two common memory exercises -- associative word-pairing tests and motor sequence tapping tasks, which involved repeatedly finger-tapping a specific sequence.
In both study nights, each participant had electrodes on their scalps.
During sleep one of the nights, each person received tACS -- an alternating current of weak electricity synchronised with the brain's natural sleep spindles. While sleeping the other night, each person received sham stimulation as placebo.
Each morning, researchers had participants perform the same standard memory tests.
The team found no improvement in test scores for associative word-pairing but a significant improvement in the motor tasks when comparing the results between the stimulation and placebo night.
"This demonstrated a direct causal link between the electric activity pattern of sleep spindles and the process of motor memory consolidation." Frohlich said.
Caroline Lustenberger, first author and postdoctoral fellow in the Frohlich lab, added that they’re excited about this because they know sleep spindles, along with memory formation, are impaired in a number of disorders, such as Alzheimer's and Schizophrenia.
“We hope that targeting these sleep spindles could be a new type of treatment for memory impairment and cognitive deficit,” she said.
Frohlich's team previously used tACS to target the brain's natural alpha oscillations to boost creativity. It showed it was possible to target these particular brain waves, which are prominent as people create ideas, meditate or daydream.
The findings are published in the journal Current Biology.
Just because you have nodded off to sleep at night does not mean your brain has stopped working hard.