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Have you ever wondered how dolphins—mammals who need oxygen to survive but reside in the uniquely oxygen-deficient ocean—sleep without drowning? Don’t worry, this is going somewhere.

In short, dolphins have an incredible adaptation wherein only one hemisphere of their brain “sleeps” at a time, leaving the other half awake to both scan for predators and control swimming to the surface for breathing. This works really well for dolphins—and birds and other unihemispheric slow-wave sleepers—but not so well in humans, who need both hemispheres to power down to feel rested.

According to The Economist, research by Yuka Sasaki and her colleagues at Brown University reveals that humans mimic unihemispheric sleep in certain instances, too—most frequently when they sleep in an unfamiliar place. The result? Feeling tired in the morning.

In the study, researchers monitored 35 young and healthy volunteers with neuroimaging techniques as they spent two nights sleeping in the (unfamiliar to them) Department of Psychological Sciences.

Dr Sasaki found that, as expected, the participants slept less well on their first night in the lab than they did on their second, taking more than twice as long to fall asleep and sleeping less overall. During deep sleep (as opposed to the lighter phases of sleep which are characterised by rapid eye movement), the participants’ brains behaved assymetrically, in a manner reminiscent of that seen in birds and dolphins. More specifically, on the first night only, the left hemispheres of their brains did not sleep nearly as deeply as their right hemispheres did.

Followup experiments showed that the “awake” left hemisphere was monitoring the sleeper’s environment for any changes.

Based upon these findings, Dr Sasaki argues in Current Biology that the first-night effect is a mechanism that has evolved to function as something of a neurological nightwatchman: to wake people up when they hear noises when sleeping in an unfamiliar environment...

There is only so much you can do to combat this evolutionary adaptation when you’re traveling for work or vacation. The effect seems to lessen after the first night—so if you need to be productive after sleeping in a foreign bed, consider going a day early and working in a buffer night.

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