The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject. It is likely that sleep evolved to fulfill some primeval function and took on multiple functions over time. (As an analogy, the larynx in all mammals controls the passage of food and air, but may have descended in humans to take on speech capabilities in addition.)

It has been pointed out that, if sleep were not essential, one would expect to find:

• Animal species that do not sleep at all
• Animals that do not need recovery sleep when they stay awake longer than usual
• Animals that suffer no serious consequences as a result of lack of sleep

Outside of a few basal animals that have no brain or a very simple brain, no animals have been found to date that satisfy any of these criteria.