By Ed Yong
Other animals can straddle the boundaries between sleeping and wakefulness. Whales, dolphins, and many birds can sleep with just one half of their brains at a time, while the other half stays awake and its corresponding eye stays open. In this way, a bottlenose dolphin can stay awake and alert for at least five days straight, and possibly many more.
Sasaki wondered if humans do something similar, albeit to a less dramatic degree. Maybe when we enter a new environment, one half of our brain stays more awake than the other, so we can better respond to unusual sounds or smells or signs of danger. Maybe our first night in a new place is disturbed because half our brain is pulling an extra shift as a night watchman. “It was a bit of a hunch,” she says. “Maybe we’d find something interesting.”
While they snoozed, team members Masako Tamaki and Ji Won Bang measured their slow-wave activity—a slow and synchronous pulsing of neurons that’s associated with deep sleep. They found that this slow activity was significantly weaker in the left half of the volunteers’ brains, but only on their first night. And the stronger this asymmetry, the longer the volunteers took to fall asleep.
The team didn’t find this slow-wave asymmetry over the entire left hemisphere. It wasn’t noticeable in regions involved in vision, movement, or attention. Instead, it only affected the default mode network—a group of brain regions that’s associated with spontaneous unfocused mental activity, like daydreaming or mind-wandering. These results fit with the idea of the first-night brain as a night watchman, in which the left default mode network is more responsive than usual.
To test this idea, Sasaki asked more volunteers to sleep in a normal bed with a pair of headphones. Throughout the sessions, the team piped small beeps into one ear or the other, either steadily or infrequently. They found that the participants’ left hemispheres (but not the right) were more responsive to the infrequent beeps (but not the steady ones) on the first night (but not the second). The recruits were also better and quicker at waking up in response to the beeps, when the sounds were processed by their left hemispheres.
This shows how dynamic sleep can be, and how attuned it is to the environment. The same applies to many animals. In 1999, Niels Rattenborg from the Max Planck Institute for Ornithology found that ducks at the edge of a flock sleep more asymmetrically than those in the safer center. “In this way, sleeping ducks avoid becoming sitting ducks,” he says. Fur seals do something similar; they sleep in the usual way on land, but at sea, they sleep on one side with the open eye looking down, perhaps to watch for sharks.
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