For questions pertaining to the physiology and pathophysiology of sleep in humans and animals (e.g., sleep stages and their EEG patterns, sleep disturbances), along with psychological effects of sleep deprivation or fatigue.
Sleep is divided up into 5 stages (stages 1, 2, 3, 4, and REM). Since 2007, NREM sleep has been reclassified into stages N1, N2, N3, with N3 being a combination of the former stages 3 and 4; however, most scientific papers are still using the old system, so I'll refer to them by the old stage names. Below, you can see a typical hypnogram, where sleep stage is plotted versus time for one night's sleep. Cycles through the night tend to progress from 1→2→3→2→REM, but can change unexpectedly if the subject is awakened.
For further information, see Sleep.