Mysteries of Sleep: what science already knows about the sleeping brain

A big brown bat sleeps 20 hours a day, hanging safely on a cave wall. A dolphin sleeps with half its brain at a time, so it never stops breathing. And the octopus, with a genome so different from ours, sleeps about as much as a human does. Every animal studied so far sleeps, and science is still trying to explain why.

Mysteries of Sleep, a 2020 episode of NOVA, the science series of the American public broadcaster PBS, travels through laboratories in the United States and the Netherlands to show what researchers can now see in the sleeping brain. Among those who speak are sleep researchers Matthew Walker, Phyllis Zee and Gina Poe.

Memories filed away at night

As early as 1924, two Cornell University psychologists noticed that students remembered more of what they had learned when they slept soon afterward. The film shows the current version of that finding: at the University of Arizona, toddlers who napped after learning made-up words got about 80% of them right the next day, against 30% for those who went hours without sleeping. The proposed explanation is that slow-wave sleep moves memories from the hippocampus, a kind of temporary drawer, to the cortex. With age these waves shrink, and Zee's team is testing short pulses of pink noise during the night to strengthen them in older adults.

Emotion, trauma and insomnia

Dutch researcher Eus van Someren shows that in people with insomnia, REM sleep is broken by brief arousals, and that the amygdala, the brain's alarm, does not settle down overnight as it does in good sleepers. Poe, for her part, found that traumatized rats that waited a few hours before sleeping were less fearful afterward, and is testing the idea with firefighters.

Cleaning, clocks and genes

The episode also covers the waste clearance the brain performs during deep sleep, described in mice in a 2013 study, the biological clock that earned the 2017 Nobel Prize in Medicine, the adenosine that caffeine blocks and mice made more resilient to sleep loss by a clock gene, BMAL1, boosted in their muscles.

Caveats, and why watch

Many results come from animals or small groups, and the scientists themselves say they are only at the tip of the iceberg. But the film is first-rate science journalism: clear, full of human stories and free of promises of a cure. It closes with simple advice from someone who studies the subject: protect your sleep the way you protect your food.