Oh, So That's Why You Never Sleep Well During a One-Night Stand

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We’ve all been there: You wake up after spending the night at a new guy’s place, and you just know you’re going to be dragging ass all day. But here’s something you might not be aware of: That groggy feeling has a name, and it’s called “first-night effect.” A new study published in Current Biology gives us some insight as to what exactly is going on.

A team from Brown University tested 35 people who were catching Zs away from home and found that one hemisphere of their brains remained more vigilant—but only on the first night. The scientists think that’s because some parts of the brain innately feel the need to “keep watch” because you’re unfamiliar with your surroundings. It’s similar to behavior dubbed “unihemispheric sleep” that researchers have seen in some animals, including whales and dolphins. (One side of their brain nods off, while the other stays active.)

To keep tabs on the subjects’ brain activity while snoozing, the scientists used a combination of advanced neuroimaging techniques and traditional polysomnography, which is a type of sleep study that monitors brain waves, blood oxygen levels, heart and breathing rates, and movements in eyes and legs. The results showed that the slow-wave brain activity that marks deep sleep wasn’t as pronounced on the brain’s left side during that first night. They also noticed that these hemispheric discrepancies only happened in the part of the brain where daydreaming takes place.

One caveat: The study's subjects were all young individuals (though no specific ages were listed in the text made available), and slow-wave activity changes with age, so the data and findings may not apply to people outside whatever range was examined.

Still, if you anticipate shacking up in a new spot, there are a few tricks you can use to sleep better: Dial down the carbs and booze and exercise in the morning.