And poised to attack.
If you've ever struggled to switch off on the first night in an unfamiliar place, there might be more at work than jetlag or cheap sheets.
A study by Brown University have found that a network in the left hemisphere of the brain "remained more active" than the other during deep sleep.
The 'first night effect' as they termed it means that half of us stays alert in case of danger when we sleep in a new place.
The laboratory experiment involved playing sounds into the right ear of 35 people which would directly stimulate the left side of the brain. Researchers found that participants were less disturbed and woke fewer times than when they played sounds to the left ear (to stimulate the right side of the brain.)
When the experiment was repeated on the following nights they found the left side of the brain didn't respond and couldn't be stimulated in the same way during deep sleep.
So 'sleeping with one eye open' isn't such a silly idea after all, unless you're attacked on night two. Then you're screwed.
H/T: Esquire.co.uk