  Sleep is one of the most noticeable and widespread phenomena occurring in
multicellular animals. Nevertheless, no consensus for a theory of its origins
has emerged. In particular, no explicit, quantitative theory exists that
elucidates or distinguishes between the myriad hypotheses proposed for sleep.
Here, we develop a general, quantitative theory for mammalian sleep that
relates many of its fundamental parameters to metabolic rate and body size.
Most mechanisms suggested for the function of sleep can be placed in this
framework, e.g., cellular repair of damage caused by metabolic processes and
cortical reorganization to process sensory input. Our theory leads to
predictions for sleep time, sleep cycle time, and REM (rapid-eye-movement) time
as functions of body and brain mass, and explains, for example, why mice sleep
\~14 hours per day relative to the 3.5 hours per day that elephants sleep. Data
for 96 species of mammals, spanning six orders of magnitude in body size, are
consistent with these predictions and provide strong evidence that time scales
for sleep are set by the brain's, not the whole-body, metabolic rate.
