The 'reason' for sleep is still somewhat of a mystery. However, ... at least part of the picture is that it is a time when short-term memories gathered during the waking period are consolidated into longer-term memory.

Rodney A. Brooks, Flesh and Machines: How Robots Will Change Us

This is how I feel about spending time with people—after being with people for long enough (hours), I start to feel uneasy, unless I get a chance to sit by myself and to start to congeal the thoughts that arose while I was in company.