Scientists have uncovered the reason why we sleep badly in a new bed – half our brain stays awake in surveillance mode.

Dubbed the “first night effect” by researchers, sleeping in a new location can often be dreadful compared to a night in your own bed.

But researchers believe a restless night can be fixed by taking your own pillow or booking a hotel room most like your own.

Experts tested 35 volunteers and found that when the participants stayed somewhere new, one hemisphere of the brain went to sleep and the other half remained “on watch".

But by the second night’s sleep, both sides of the brain returned to normal.

Sleep scientist Masako Tamaki told the Guardian: “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.

"It’s like a safeguard.