"because it is part of a mechanism that serves to synchronise wake/sleep cycles among different members of the social group. (Reference:Provine et al,Ethology,1987,v.76,p.10", according to Dr A.T.Chamberlain, Dept.Human Anatomy,University of Liverpool.

However , it's only a hypothesis.

It's interesting that thread is called 'science stuff we believe', rather than 'science stuff we know'.

Ivor