The phenomenon is so familiar to sleep researchers that it has its own name: the first night effect. For some reason, when we tuck ourselves up under a new roof, we often have a troubled night and wake up feeling groggy and tired.

Scientists have studied sleeping brains for half a century in the hope of understanding the first night effect. And in research published today, a US team has made fresh headway. When we stay somewhere new, our brains seem to spend the first night in surveillance mode. While one hemisphere goes to sleep, the other half of the brain remains on night watch.

“If we don’t know whether a room is safe to sleep in, then we will have this night watch system so we can detect anything unusual,” said Masako Tamaki, a sleep scientist at Brown University in Providence, Rhode Island. “It’s like a safeguard.”

Should the findings be confirmed by further studies, the first night effect may turn out to be the human equivalent of birds sleeping with one eye open - and one half of the brain awake - to ensure they are not eaten by night time predators. Dolphins use a similar trick, and alternate sleep between the two halves of the brain, so they can rest while keeping a constant eye out for sharks.

To investigate the first night effect, Tamaki and her team turned to a suite of sophisticated brain imaging techniques. They used magnetoencephalography (MEG), structural MRI and polysomnography, which is a combination of muscle tone and other measurements, to monitor in detail how 35 people slept in the laboratory.

On the first night, the scans revealed that the right halves of the participants’ brains fell asleep as normal, but the left hemispheres remained more active. The researchers then tested how people responded to sounds as they slept. They began by playing in low pitched tones every second. The brain largely ignored these, but when the researchers added the occasional unusual sound, such as a high-pitched tone, the left side of the brain jolted into action, and people were more likely to wake up. The finding, said Masako, suggests that the brain’s night watch system is alert to unusual sounds which could represent a threat. The effect vanished on the second night, according to a report in Current Biology.

The scientists are not sure whether the left hand side of the brain is always more alert during the first night effect. They recorded brain activity for only 90 minutes as people slept, so it is possible that, like the dolphin, the brain switches its night watch system from one side to the other while the person sleeps. “There’s certainly a possibility that the vigilant hemisphere changes over the night,” Tamaki said.

People who sleep in new places all the time might not experience the problem very much, as their brains get used to the variation. “Human brains are very flexible,” said Yuka Sasaki, a co-author on the study. But those who do suffer might reduce the effect by taking their own pillow with them when they travel, or by staying in places that have similar rooms, she added.

Derk-Jan Dijk, director of Surrey University’s Sleep Centre, said the study builds on work that began in 1966 when researchers in Florida performed the first electroencephalograms (EEG) to investigate the phenomenon. “The first night effect is a real effect. You sleep in a new environment and you don’t sleep as well as you do in the bedroom you sleep in every night. It’s been known about for a long time and is used as a model of sleep disruption,” he said. “What these researchers show is that during the first night, one side of your brain doesn’t sleep as deeply as the other side of your brain.”

The findings could shed light on an issue that comes up with some insomniacs who are studied in sleep research centres. Patients may say they did not sleep all night, when EEG recordings of their brain activity suggest that they did. “If you do high density EEG, you can sometimes see signs of arousal in some brain areas of these patients,” said Dijk. “So this finding adds to the data that sleep is not a complete global phenomena, there are local aspects to it. The brain can be locally awake, and maybe this is maybe what is happening in the first night effect.”

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