Until recently, I didn't know that ducks such as Mallards are able to sleep with one eye open. They are literally half asleep! The scientific term is uni-hemispheric sleep. It means that one hemisphere of the brain sleeps while the other remains awake and alert for predators. This can be observed with a group of ducks. Those in the centre often sleep with both eyes closed, while those on the outside sleep with one eye open. A similar phenomenon has been observed in humans. When sleeping in unfamiliar places, the right ear and the left hemisphere of the brain remain more active and able to sense danger. Sleep is shallower and awareness of noise is significantly higher.