A

n animal sleeps to avoid feedback. In this manner:
External input causes processing which in turn generates more processing because the result of the former is internal input. The processing result must then be prevented from causing additional internal processing or the entity must shut down external input / output and become 'introspective'.

An animal then has three principle brain functions:

To take in information from the external environment (~1/3 of the time) To process that data for the purpose of generating output into the environment (~1/3 of the time) To continue integrating new data into the context resulting in improved processing ability (~1/3 of the time).

Because new data generates more internal input than old already known data, infants require more sleep. Context is not yet established and there is no significant output. The infant is then primarily taking in information and organizing it internally (and little else) requiring half time (or more) sleep cycles.
Conversely elderly people require less sleep because the days experiences do not significantly require alteration of their already developed context.

	Web ebtx.com