What changes, to your question, is the ratio of non-REM to REM within that 90-minute cycle as you move across the night ... In the first half of the night, the majority of those 90-minute cycles are comprised of lots of deep non-REM sleep ... Once I push through to the second half of the night, now that seesaw balance changes, and instead, the majority of those 90-minute cycles are comprised either of this lighter form of non-REM sleep (stage 2 non-REM sleep) and much more, increasingly more, rapid eye movement sleep.