Sleep time is spent as follows: Stage N1: 2 - 5% Stage N2: 45 - 55% Stage N3: 13 - 23% REM: 20 - 25% Etiology Some disorders can cause either insomnia or EDS (and sometimes both), and some cause only one or the other (see table Some Causes of Insomnia and Excessive Daytime Sleepiness).