Ů��С��Ƶ

This is why you feel so awful after a bad night’s sleep – your brain is jammed with yesterday’s news.

at the University Medical Center in Freiburg, Germany, and his team examined the brains of 20 people after they’d slept well, and after a night of disruption. As well as performing worse in a memory test, they found that, after a bad night’s sleep, people had higher levels of theta brainwaves, and it was easier to stimulate their brains using magnetic pulses.

These are both signs of stronger connectivity between neurons. As we form new memories, the connections between our neurons – called synapses – strengthen, building up over the course of a day. “The overall strength of connections between neurons increases with time awake, and eventually reaches a level of saturation after prolonged wakefulness or sleep deprivation,” says Nissen.

His team’s findings support the theory that sleep serves to weaken memory connections, making way for new ones. “Without this synaptic downscaling, the brain loses the capacity to form novel connections, impairing the encoding of novel memories,” says Nissen.

Active sleep

In July, at the University of Wisconsin-Madison unveiled data from thin slices of brain taken from mice that showed that synapses in the brain are smaller after sleep. “In mice, we can look directly at synapses at the finest ultrastructural level, which is not possible in humans,” says Tononi. But Nissen’s data is consistent with these findings in rodents.

“Our study highlights the importance of sleep, and the notion that sleep is a highly active brain process, not a waste of time,” says Nissen.

He believes a better understanding of what happens while we sleep could be used to boost the performance of sportspeople or musicians, and possibly to treat conditions like depression and stroke.

Journal reference: Nature Communications, DOI: 10.1038/ncomms12455

Read more: Mystery of what sleep does to our brains may finally be solved