There's nothing worse than not being able to settle down to sleep in an unfamiliar room.

That sense of uneasiness can keep you lying awake at night for hours, whether it be in a hotel room for a work trip, on a friend's couch, or in your significant other's place you've never stayed at before. Wherever it is, it's horrible and we despise it. According to Mental Floss, there's actual science behind your eyes still being wide-open at 2am in your work trip hotel room. Published in the journal Current Biology, it turns out that the reason comes down to something quite primal. Constant, animal-like vigilance. Some animals sleep with one eye open, to keep an eye out for predators, like beluga whales and bottlenose dolphins. It's called unihemispheric slow-wave sleep (USWS), and apparently scientists have found humans do this too (or USWS-lite). FNE, or First Night Effect is that feeling of unfamiliarity and the scientists wanted to identify the cause of it. 35 volunteers were taken for a sleep study of two nights sleep (with one week break in between). Machines measured their heart rates, blood oxygen levels, breathing, eye and leg movements, as well as activity in both sides of the brain. Scientists focused on slow wave activity which monitors depth of sleep, and they found that the subjects' brains were far less sensitive to strange sounds that could be a potential threat, suggesting they had become used to their environment. Co-author Yuka Sasaki concluded that"our brains may have a miniature system of what whales and dolphins have", but frequent travellers may be able to override this primitive instinct by repeatedly staying in different places. The more you know.