Sleeping in a strange bed is often a less-than-restful experience because one half of the brain remains "on watch", a study has shown.
The "first-night effect" is well known to hotel-hopping business travellers and sleep researchers, who always take account of it in their experiments.
Now one group of scientists has attempted to find out why people tend to sleep so badly during their first night in unfamiliar surroundings.
A range of brain scanning techniques revealed a surprising discovery. On the first night - and only the first - the left side of the brain remains much more "awake" than the right side.
The effect is seen during deep "slow-wave" sleep and causes the left brain hemisphere to be especially sensitive to sound.
As a result, any kind of unexpected noise is liable to wake the sleeper.
Scientists believe that in a strange and potentially hostile environment, our vigilant brains ensure we remain at least partially alert at night.
And they suggest a possible solution for weary hotel guests: travel with your own pillow. The idea is to lull the brain into a sense of security that ensures a good night's sleep.
Psychologist Dr Yuka Sasaki, a member of the team at Brown University in Providence, US, said: "In Japan they say, 'if you change your pillow, you can't sleep'. You don't sleep very well in a new place. We all know about it."
The researchers used different methods to measure the brain activity of 35 volunteers who were asked to spend two nights in their laboratory, a week apart.
They found that on the first night, the left hemisphere of the participants' brains remained more active than the right.
Stimulating the left brain hemisphere by playing bleeping sounds in to the right ear was more likely to wake the volunteers than if the same action was reversed to stimulate the right brain side.
The scientists wrote in the journal Current Biology: "To our best knowledge, regional asymmetric slow-wave activity associated with the first-night effect has never been reported in humans.
"The present study has demonstrated that when we are in a novel environment, inter-hemispheric asymmetry occurs ... as a night watch to protect ourselves."
Whales and dolphins also sleep with one side of their brains awake, said the researchers. The most likely reason is that they need to resurface regularly to breathe, even during sleep.
The scientists used three different techniques to scan volunteers' brain activity: electroencephalography, magnetoencephalography and magnetic resonance imaging (MRI).
They point out that their findings just answer part of the riddle. Only the first phase of slow-wave sleep, which recurs several times during the sleep cycle, was measured. Whether the left hemisphere keeps watch all night, or works in "shifts" with the right side of the brain is not known.
"It is possible that the surveillance hemisphere may alternate," said Dr Sasaki.