Ostriches sleeping, with REM and<unk>  phases<unk>  <unk> , often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep. Initially,<unk>  consisted of both Stage 3, which has 20–50 percent delta<unk>  activity, and Stage 4, which has more than 50 percent delta<unk>  activity.
Slow-wave sleep
Non-rapid eye movement sleep
Rapid eye movement sleep
Sleep onset
Postictal state
Biphasic and polyphasic sleep
Twilight sleep
Free-running sleep