Sleep stages
Sleep comes in steps like a staircase: you go down from light sleep into deep sleep, then move into dreaming sleep. This round repeats several times a night.
A little more detail
In AASM scoring, 30-second epochs are marked as W (wake), N1, N2, N3 and R. Stages 3 and 4 of the pre-2007 Rechtschaffen and Kales system have been merged into N3.
Source: Silber MH, Ancoli-Israel S, Bonnet MH et al. The visual scoring of sleep in adults. J Clin Sleep Med. 2007;3(2):121-131. doi:10.5664/jcsm.26814