While You Sleep, Your Brain Sorts the Day's Memories

If you have ever pulled an all-nighter before an exam, you probably also remember how foggy your head felt the next day. Sleep was long thought of as the brain’s “time off,” but decades of research paint the opposite picture. While we sleep, the brain sorts the information that came in during the day, replays it, and rewrites it in a form that will last.

A night’s sleep is made of several cycles

Sleep is not a single state. Over one night, a cycle that descends from light sleep into deep slow-wave sleep and then rises into dream-rich REM sleep repeats roughly four to six times. Each cycle usually lasts about 90 minutes. Deep slow-wave sleep dominates the early part of the night, and REM sleep takes up a larger share toward dawn.

The hippocampus “replays” the day

New experiences from the day are first stored temporarily in the hippocampus. You can think of the hippocampus as a notepad that records quickly but doesn’t hold much. During deep slow-wave sleep, the hippocampus replays, in compressed form, the sequences of neurons that were active during the day, and these signals are timed to line up with the slow waves of the cerebral cortex and with short bursts of brain activity called sleep spindles. Through this process, memories are gradually moved into more stable networks in the cortex. That is the widely accepted explanation today (Rasch & Born, 2013).

In animal studies, hippocampal cells that fired in sequence while a rat ran through a maze were later observed firing again, rapidly and in the same order, while it slept. Experiments with people have also repeatedly found that those who sleep after learning something retain it better than those who stay awake for the same amount of time.

So what do we lose when we cut sleep?

Because deep slow-wave sleep is concentrated early in the night and REM sleep around dawn, going to bed late and waking up early each cost you a different part of sleep. What matters is not just total hours but the structure of sleep, too. The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults regularly sleep 7 or more hours a night for their health (Watson et al., 2015).

Today’s practice

1. Protect your sleep on days you learn. The more you learned something new or studied for an exam that day, the more it helps your memory to sleep as usual rather than cut back.
2. Review for 10 minutes before bed. Lightly going over the key points of the day before you sleep makes it more likely that they’ll be on the list of information processed while you sleep. But pushing back sleep to cram in new material late at night backfires.
3. Fix your bedtime. Going to bed at a regular time, such as around 11 p.m., reliably secures the early part of the night when deep sleep is most abundant.
4. Dim the lights from an hour before bed. Bright light delays the release of melatonin and pushes back the time you fall asleep. See also our article on morning sunlight.

If someone tells you that your breathing seems to stop during sleep, or if you feel very sleepy during the day despite sleeping enough, it’s a good idea to see a sleep specialist.

References

• Rasch, B., & Born, J. (2013). About sleep’s role in memory. Physiological Reviews, 93(2), 681–766.
• Watson, N. F. et al. (2015). Recommended amount of sleep for a healthy adult: A joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep, 38(6), 843–844.