Explaining many sleepless nights when on the road, scientists discover that when sleeping in a new place, half your brain stays in a near awake status, apparently just in case one of those pesky saber toothed cats jumps through the window to get at you.

Sleep scientists have regarded the First-Night-Effect (FNE) in humans as a regular sleep disturbance for some time, but they’ve never fully grasped how it works. So sleep scientist Masako Tamaki and her colleagues took it upon themselves to find out why. Using advanced neuroimaging techniques, they carefully analyzed a number of snoozing brains.

Strangely, they found that the sleeping brains showed asymmetrical patterns of sleep activity, with one hemisphere humming along while the other slept. And while the sprightly hemisphere wasn’t fully awake, it was much more active than the other—even responsive to external stimuli. Subjects in the study experiencing FNE, for example, were jolted awake by “deviant” sounds. A creaking door perhaps. Or a shrieking animal. For most of the subjects, the night watchman hemisphere of their brain was the left side, for inexplicable reasons.

Inexplicable unless you are a member of a band of proto-humans following the herds across the plains of Central Asia after the last ice age.  The ones that survived the night attacks of the Neanderthals, or the lions, were the ones that awoke first and got away, while their heavier sleeping friends became dinner.

It's another genetic echo from when we were living in much more primitive and dangerous conditions, which, in biological terms, just wasn't that long ago.