Ever wake up groggy after your first night in a hotel? Your brain may be keeping watch for danger.
New research suggests that the parts of the brain do not go into "sleep mode" during the first night of sleep in a new environment.
Sleep researchers have long known about "first-night effect," and consider it a temporary sleep disorder. Even if the room is the right temperature and the bed is comfortable, sleepers often report waking up groggy and less rested than they would at home.
The phenomenon is so common that scientists running sleep studies frequently throw out the first night's data. The condition is only temporary — it typically recedes by the second night. Still, it can be the bane of many a business traveler aiming for eight solid hours before a presentation.
A group of researchers from Brown University and the Georgia Institute of Technology wanted to know why the "first night effect" strikes. They took images of sleeping brains in two experiments and found something surprising.
They found that the brains of sleeping people with first night effect were not showing the "slow wave activity" brains normally show during deep sleep. One hemisphere of the brain appeared to still be awake.
The researchers think an explanation for the phenomenon may lie in our evolutionary past.
"It was quite surprising" said Masako Tamaki, one of the study's co-authors, told CNBC. She said similar behavior has been observed in other animals, especially mammals and birds, and it might be related to protection in risky situations.
"When people sleep in a new room, we are not really sure if the room is safe or not to sleep deeply. It could be possible that we keep this one brain hemisphere vigilant so that we can monitor and detect something unusual in our surroundings."
The researchers also ran an experiment that showed how that slightly more vigilant part of the brain is able to induce wakefulness much faster when it registers an "unusual signal" from the environment.
The researchers played "unusual sounds" while patients were sleeping, and found they were able to react to them more quickly upon waking up.
They published their findings Thursday in the journal Current Biology.
So what should a frustrated sleeper do to combat the effects of the first-night effect? The researchers have not yet tested possible remedies for the condition. But there may be a few ways to mitigate its effects.
Since the condition seems to last for only one night, it might help to arrive two nights before any important events, such as a business presentation, Tamaki said.
It also might help to bring familiar objects on a trip, such as a pillow. Sleepers can also try to sleep in the same places when traveling.
And don't stress over the possibility of looming grogginess, Tamaki said. Worrying is terrible for sleep, so you may be better off just giving in to the phenomenon.