Why You Feel Groggy When You’re Not In Your Own Bed

New study suggests that when you're away from home, a part of your brain stays awake all night—keeping watch

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Apr 25, 2016 at 9:42 AM ET

No matter how tired you are, you just can’t seem to nab a good night’s sleep while staying in a hotel or another strange bed. Now, a new study in Current Biology explains why: evolution, of course. The study suggests our brains are hardwired to stay partially alert throughout the night when we’re in unfamiliar territory to keep watch for danger.

“We know that marine animals and some birds show unihemispheric sleep, one [brain hemisphere] awake and the other asleep,” said coauthor Yuka Sasaki of Brown University, in a press statement. Sasaki speculates that, when in unfamiliar settings, “our brains may have a miniature system of what whales and dolphins have.”

More Confirmed: Lack Of Sleep Is Linked To Catching A Cold

That brains can remain attuned to the environment even while asleep is hardly news. Brain scans have shown that ducks sleeping in vulnerable positions near the edge of the flock are only partially asleep, always on the lookout for predators, while fur seals tend to sleep on their sides while at sea, with one eye gazing downward into the water to detect sharks—so they literally sleep with one eye open.

But we humans tend to sleep with all of our brains—at least, when we’re in our own beds. Researchers have long suspected, however, that something funny happens neurologically when we sleep in strange places. In fact, sleep scientists traditionally discard the data from the first night that a subject sleeps in the lab due to a phenomenon known as first-night effect, because nobody sleeps well on their first night away from home.

In an effort to better understand first-night effect, Sasaki and colleagues invited 11 volunteers to sleep in their laboratory while an enormous medical scanner and a host of electrodes measured their brain activity. The researchers were specifically looking for slow-wave brain activity, which is associated with deep sleep. They found that, on the first night in the lab, subjects had significantly weaker slow-wave activity in the left half of their brains, indicating shallow sleep or even slight wakefulness, and that only in subsequent nights did the left hemispheres of their brains gradually began to sleep more deeply.

Interestingly, the specific regions of the left brain that stayed awake on that first night were not involved in vision or movement, but comprised what is known as the default mode network, a cluster of neurons responsible for spontaneous, unfocused mental activity. In other words, the sort of neurons you’d want around if you suddenly needed to make a run for it. In fact, the researchers found that subjects whose left brains were awake in “default mode” may have been groggier the next day—but they were also able to wake up more quickly when an alarm went off. Broadly, the findings suggest that humans may have a similar survival technique to those seen in birds and marine mammals—when we’re in a strange place, part of our brain remains primed to make an escape even while we sleep.

But some scientists are skeptical of the results. Luigi Degennaro of Sapienza University in Rome told The Atlantic that the small sample size (a mere 11 volunteers) should be a deal breaker—it’s simply not enough to draw meaningful conclusions. Furthermore, Sapienza derided the team’s methodology, as The Atlantic puts it, “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.” 

Still, Sasaki and colleagues believe they may be onto something. Sasaki says that she intends to recruit more participants, and perhaps even attempt to use magnetic stimulation to knock out the “awake” part of the brain on the first night in the lab and test whether or not sleep improves. Until then, however, Sasaki advises travelers who just can’t fall asleep to try bringing their own pillows along for the journey, or staying in similar hotels at each destination, so that the brain feels comfortable enough to go to sleep entirely.

“Human’s brains are very flexible,” she says. “Thus, people who often are in new places may not necessarily have poor sleep on a regular basis.”