A short afternoon nap may help restore the brain's flexibility to take in and respond to new information after hours of wakefulness. Digest
As we learn, the connections between brain cells that are being used become stronger. But after many hours without rest, this overall buildup can leave the brain with less room to make new, specific changes. Scientists have long suggested that sleep helps the brain stay flexible by easing back this buildup, but it has not been clear whether a short daytime nap is enough to do the same.
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Researchers brought 20 healthy young adults into a sleep lab for two experimental sessions. In one session, they were given a one-hour afternoon sleep opportunity. In the other, they stayed awake for the same amount of time. After each session, scientists used transcranial magnetic stimulation, which sends brief magnetic pulses through the scalp, to see how strongly the motor cortex (movement area of the brain) could trigger a small electrical response in a hand muscle. They also recorded brain activity with electroencephalography (EEG), focusing on theta waves, a signal that rises the longer we stay awake and is used as an indirect sign that overall communication between brain cells has become stronger. Finally, they delivered a mild electrical pulse to a nerve at the wrist while also stimulating the motor cortex. When carefully timed together, this pairing can temporarily make communication between brain cells stronger. The change in response after stimulation serves as a lab measure of how easily that brain area can strengthen its connections.
- Participants slept about 43 minutes on average during the nap, almost all of it in non-rapid eye movement (non-REM) sleep and mainly in the deeper N2 and N3 stages.
- Before any training-like stimulation, the brain needed a slightly stronger magnetic pulse to activate the hand muscle after the nap. This suggests that the overall strength of communication between brain cells was lower after the nap than after staying awake.
- Theta brain waves increased after staying awake but not after the nap, suggesting that time awake builds up overall communication between brain cells, while a nap is associated with lower levels of that buildup.
- Researchers then tried to strengthen specific connections using the paired stimulation. After this training-like procedure, the muscle response increased after the nap but not after staying awake, suggesting that the tested brain area was better able to strengthen its connections after the nap.
- Seventy-five minutes later, 80% of those who napped showed a strengthening response, compared with 55% of those who stayed awake.
Together, the findings suggest that napping helps prevent the brain's connections from becoming so built up during the day that they lose flexibility. As we stay awake, connections between nerve cells gradually grow stronger, and if that buildup continues, the brain may become less able to make further changes. A nap appears to ease this buildup, restoring the brain's readiness to respond to new stimulation.
The study included a small group of participants and did not use a placebo-style stimulation condition, making it harder to tell whether the differences were truly caused by the nap or by natural differences in how individuals respond to this type of stimulation. Still, the results suggest that a short daytime nap can recalibrate measures of plasticity in the brain. In this clip, Dr. Matthew Walker describes how napping facilitates and reinforces learning in infants.