She invited 11 volunteers to spend a few nights at her laboratory. They slept in a hulking medical scanner that measured their brain activity, while electrodes on their heads and hands measured their brain waves, eye movements, heart rate, and more. “The scanner has a bed that could go completely flat, and we put a lot of pillows and towels to make it comfortable,” says Sasaki. “It was a little restricted but people could still sleep.” And sure enough, they took longer to fall asleep and slept less deeply on the first night.

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.

“It’s very exciting to see that researchers have now found something similar in humans,” says Rattenborg. “It seems reasonable to speculate that, as in ducks and seals, this is an adaptive response that provides us with some protection when sleeping in novel environments, wherein we have limited information about potential threats.”

Lino Nobili from Niguarda Hospital in Milan adds that these results fits with a “relatively new view of sleep” as a patchwork process, rather than a global one that involves the whole brain. Recent studies suggest that some parts can sleep more deeply than others, or even temporarily wake up. This might explain not only the first-night effect but also other weird phenomena like sleepwalking or paradoxical insomnia, where people think they’re getting much less sleep than they actually are.