When you check into a hotel room or stay with a friend, is your first night of sleep disturbed? Do you toss and turn, mind strangely alert, unable to shut down in the usual way? If so, you’re in good company. This phenomenon is called the first-night effect, and scientists have known about it for over 50 years. “Even when you look at young and healthy people without chronic sleep problems, 99 percent of the time they show this first-night effect—this weird half-awake, half-asleep state,” says Yuka Sasaki from Brown University.

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.”

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.

But other sleep scientists are more skeptical. Luigi Degennaro from Sapienza University in Rome derided the small numbers of volunteers, as well as the team’s methods. For example, rather than focusing on the default mode network (DMN) from the off, he says they should have looked at all brain regions where slow-wave activity differs between the left and right halves, and then checked if these corresponded to the DMN or other networks. He also says that the team needed to account for factors like handedness or gender, which could have contributed to asymmetric brain activity, beyond any hypothetical night-watch effect.

Along similar lines, Vladyslav Vyazovskiy from the University of Oxford wants to know if the rooms had asymmetric sources of light or sound, or if people slept on one side or the other. “Was the asymmetry related to the environment or sleep posture in any way?” he wonders.

But Sasaki doubts that such factors are important. In the latter experiments, the volunteers used earbuds, so any sounds came from inside their ears rather than from some speaker in the room. And throughout the study, “the asymmetry only occurs on the first night,” she says, while other factors like handedness and gender didn’t change.

The study’s small sample size is a real weakness though, and perhaps an unavoidable one given how expensive brain-imaging techniques can be. To confirm the night watch hypothesis, Sasaki now wants to use weak electric currents to shut down the left default mode network to see if people sleep faster in new environments. That would certainly support her idea that this region is behind the first night effect.  

It won’t help people sleep better in new places, though. To do that, Sasaki tries to stay in the same hotel when she travels, or at least in the same chain. “I’m flying to England tomorrow and staying at a Marriott,” she says. “It’s not a completely novel environment, so maybe my brain will be a little more at ease.”