It’s a familiar conundrum: whether to sleep over or not, after a one night stand. Once you’ve committed to it, the night is spent tossing and turning. The morning finds you unrested, and grumpy as all hell. A team of sleep researchers think they might know why. In a new study, they conclude that it’s because, when you sleep in a strange place, one hemisphere of your brain stays up to keep watch.
It’s called the “first night effect,” and has long been viewed as a typical sleep disturbance, although it’s never been fully understood.
“Participants for a sleep study [often] indicate poor sleep during the first experimental sleep session,” said study author Masako Tamaki, professor of Cognitive, Linguistic, and Psychological Sciences at Brown University. “Sleep quality usually improves significantly in the second session,” she told me, “when the environment has become no longer new to the participants.”
Because of this, sleep scientists will actually throw out data from the first night a person sleeps in the lab, and instead focus on data from the second sleep session on.
In this new study, published in Current Biology on Thursday, Tamaki and co-authors Ji Won Bang, Takeo Watanabe and Yuka Sasaki provide a possible explanation for this disturbance, one that was reflected in their subjects’ brain activity.
To find out what might cause the “first night effect,” researchers took brain scans of their subjects to get a better look at what was happening inside a sleeping brain.
During that first night of sleep, they saw something unexpected: one hemisphere of the brain showed more activity than the other. In their study, this always presented as the left side, but these sleep scientists still aren’t sure why.
Tamaki told Motherboard that it’s possible the left hemisphere is more active than right in the first cycle of sleep—which was the focus of their study—and that the right could become more active later on. But more research is needed to clarify the process through the full night of sleep and, if this finding holds up, they hope to get a better understanding of why the left side is tasked with staying awake. For now, they can’t say for sure.
It’s not the first time this phenomenon has been seen, but it is the first time it has been studied in humans.
“Some animals seem to indicate a unique sleep state, which is called unihemispheric sleep, where one brain hemisphere shows wakefulness and the other shows deep sleep,” Tamaki told Motherboard. “It has been suggested that this might be related to increased alertness to watch out for predators.”
Unihemispheric sleep has been seen in marine animals (like whales and dolphins) and some birds. Now it looks like human brains might be doing something similar.
As for what’s next, “we are curious about how we could counteract the effect, or to test how this would impact other functions of sleep,” said Tamaki. “It would be also interesting to find out whether there is any relationship with temporary sleep disturbance and chronic sleep disorders.” And it could prove hugely beneficial to some groups, like the military, where soldiers who are deployed often find themselves sleeping in new locations.
It also helps us understand one reason—among others, quite possibly—that you feel so shitty in the morning after a one night stand.