You sleep badly in hotels because half your brain watches

The first night in a hotel you sleep terribly, and the second, in the same bed and with the same noise from the corridor, you sleep well. In 2016 Masako Tamaki's team at Brown University published why: on that first night only half the brain fully sleeps. The other half stays on watch.

They measured it by having volunteers sleep two nights running in the laboratory, with the activity of each hemisphere recorded separately. On the first night, the left hemisphere had less deep sleep than the right, reacted more to the odd beeps slipped to the sleeper, and woke them sooner with them. On the second night, with the same beeps and the same bed, the difference had gone.

The phenomenon had a name from the sixties onwards — the first-night effect — and was put down to a nuisance of the laboratory: people sleep badly when you plug them into cables. What was not known is that it consisted in this, half a head standing guard in a place it has not yet decided is safe.

Dolphins and many birds sleep like that all their lives, with one hemisphere awake in shifts, which is why they can swim or fly asleep. We do it one single night for each new place, and that is why on the second nobody hears the lift.