"We have recently reviewed the genetic, cellular and subcortical mechanisms that control the circadian rhythms of sleeping and waking, as well as the ultradian regularity of alternating rapid ey movement (REM) and non-REM (NREM) stages of sleep."

Pace-Schott, E.F et al. (2002)
The neurobiology of sleep: genetics, cellular physiology and subcortical networks
Nature Review Neuroscience 3, 591-606
=> not-available
   <A little more detail could be got via BiomedExperts already registered, though>