Oddly, the ratio of non-REM-to-REM sleep within these 90-minute cycles changes across the night, regardless of when you go to bed. Early in the night (between the hours of 11 p.m. and 3 a.m.), the majority of those cycles are comprised of deep non-REM sleep and very little REM sleep. Yet, in the second half of the night (the hours between 3 a.m. and 7 a.m.), this balance changes; the 90-minute cycles are comprised of more REM sleep together with a lighter form of non-REM sleep. Because there exists a greater propensity for deep non-REM sleep earlier in the night, someone who sleeps from 9 p.m. to 5 a.m. (eight hours total) will have a different overall composition of sleep with more non-REM than someone who sleeps from 3 a.m. to 11 a.m. (also eight hours total) and so is likely to experience more REM.