Physiology. In 1937 Loomis et al. classified sleep into five different stages, A to E, based on EEG characteristics. However they failed to identify REM sleep (rapid eye movement sleep). Stage A: drowsy sleep; B: sleep onset C: light sleep; D: intermediary sleep and E: deep sleep. Stage B corresponds to sleep stage 1 of the contemporary classification, stage C to stages 2, 3 and 4.