Maybe you’re away on a business trip and you’ve got a big presentation in the morning. Maybe it’s your first night in a new home after a long day hauling boxes. Whatever the circumstances, you could really use a good night’s rest—but, given that you're sleeping in a new place, that may be easier said than done. Now, a team of scientists at Brown University say they’ve found a cause for this first-night effect: constant, animal-like vigilance. They published their findings in the journal Current Biology.

Sleep is something of a puzzle for scientists. Most animals do it, but it’s not entirely clear why it’s necessary. In survival terms, it’s pretty inconvenient for an animal to be off its guard for several hours every day. But rather than evolving to live without rest, some animals have developed the ability to literally sleep with one eye open. Bottlenose dolphins, southern sea lions, domesticated chickens, and beluga whales are among species that practice unihemispheric slow-wave sleep (USWS), in which just one half of the brain sleeps at a time.

You can see this yourself in a line of snoozing ducks: the duck at the end of the line will have its outward-facing eye open. That eye is linked to the brain hemisphere that’s still awake. That way, even in sleep, the sight of a predator could trigger alarms in the brain, cueing the duck to take action.

This may look like an angry pirate, but it’s actually a young house sparrow in USWS. Image credit: Hussain Kaouri via Wikimedia Commons // CC BY-SA 4.0

As you can imagine, this vigilant half-sleep is a real asset in dangerous and unpredictable environments. Unfortunately, your brain might count hotel rooms and new apartments as dangerous. That’s right: Scientists have found USWS in people. Or, rather, they’ve found what amounts to USWS Lite.

Sleep researchers are well aware of the first-night effect (FNE), and frequently throw out the results from a sleep study subject’s first night in the lab. Rather than working around the FNE, a team of researchers decided to identify its cause. They recruited 35 healthy volunteers and brought them into a sleep lab for two nights of sleep with a one-week break in between. The volunteers were hooked up to machines that measured their heart rates, blood oxygen levels, breathing, eye and leg movements, as well as activity in both sides of the brain.

The scientists focused on slow-wave activity (SWA), a type of brain behavior that can indicate how deeply someone is sleeping. They looked at SWA in four different brain pathways in both sleep sessions, tracking how sleep depth was affected by disturbances in the room.

They weren’t looking for differences between the brain hemispheres, but they found them. On the first night of sleep, subjects consistently showed more wakefulness in the left half of their brains. The left hemisphere was also more sensitive to strange (and thus potentially threatening) sounds. One week later, when the subjects returned to the sleep lab, there was more symmetry in the subjects’ brain activity, suggesting they had become accustomed to the now familiar environment. Their SWA showed equal levels of wakefulness, or lack thereof, in both brain hemispheres.

While the study results suggest we are participating in USWS, co-author Yuka Sasaki says in a press statement that "our brains may have a miniature system of what whales and dolphins have."

Sasaki noted that frequent travelers may subconsciously train their brains to bypass the FNE. Our brains are “very flexible,” she said. “Thus, people who often are in new places may not necessarily have poor sleep on a regular basis."

The team’s future experiments will include trying to shut off the FNE so people can get a better (first) night’s sleep.