One half of the human brain remains on alert the first time you sleep in a new place, perhaps accounting for the poor sleep often reported by travelers, a new study has shown.
A number of animals, including birds and marine mammals, are known to rest one hemisphere of their brains at any one time so that they can remain half awake in order to spot danger, but humans weren't previously included on this list.
Now, a study by Yuka Sasaki and her colleagues at Brown University in the US has found that on the first night sleeping in a new location, the left hemispheres of a group of adult volunteers showed significantly different patterns of brainwaves compared with the opposite side of the brain or the left hemisphere on a subsequent night's sleep.
The subjects also showed significantly greater responses in their left hemispheres to external noises presented as they slept, and were much more susceptible to arousals in their left hemispheres on their first night in foreign surroundings.
Told to tap their fingers if they were awoken by the night time noises presented to them, the subjects also showed significantly faster reaction times on the first night of the study compared with the second night.
The altered pattern of brain activity seen in the volunteers was manifest in changes to what is dubbed "slow wave activity" - SWA - which is a characteristic of deep sleep and arises from a brain network called the "default mode network".
This is a collective of brain regions which exchange information during periods of mind wandering, or while we play out in our minds an upcoming event.
The new results, published this week in Current Biology, suggest that the default mode network may also have a role as a "night watchman", keeping one half of the brain ticking over as we slumber in unfamiliar surroundings so that we can remain more attuned to danger.
In the present study it was always the left hemisphere that seemed to play this role, which might reflect the relative dominance of the left side of the brain in most people.
The researchers also acknowledge though that they looked only at the first phase of sleep in their volunteers, so it's possible that people do what whales do, and alternately switch between hemispheres as the night progresses.