This afternoon, I was distracted by a honking flock of geese. Whether they were returning from migration in southern latitudes is to be determined, but their presence certainly reminded me of a unique physiological phenomenon present in most bird species: unihemispheric slow wave sleep. Through unihemispheric slow wave sleep, birds and aquatic mammals such as seals and dolphins are enabled to remain awake across their lifetime.
How? As one side of the brain sleeps, the other side is awake. This unique phenomenon is also the epitome of contralateral processing within the brain; the idea that the both hemispheres of the brain are equally neccessary to execute some sensory or motor task. In lieu of contralateral processing with unihemispheric slow wave sleep, if the right side of the brain is awake, then visibilty arises from the left eye, or if the left side of the brain is aleep, then the right eye is closed.
With regards to the adaptive purpose of unihemispheric slow wave sleep, it facilitates quicker escapes from predators, and enables mothers to remain vigilant of young. In fact, if you walk through a park at night and observe ducks sleeping, you will notice that the ducks around the perimeter of a “resting” flock have the awake eye facing towards the surrounding environment, while the interiorally-located ducks have both eyes closed. Note: unihemispheric sleep doesn’t occur all the time! Even birds have dreams, I mean, REM sleep, of course. Below is a picture of the brain activity measured through electoencephalography that depicts the differentiation between high frequency waking brain activity in one hemisphere and low frequency sleeping activity in the other hemisphere.