The ratio of non-REM to REM [sleep] within each 90-minute cycle as we move across the night changes ... In the first half of the night, the majority of those 90-minute cycles are going to be comprised of lots of deep, non-rem sleep (stages three and four non-REM sleep) and very little REM sleep, but as you push through to the second half of the night, now the brain has a shift in its taste preference for what it wants to feast on at the finger buffet of sleep stages. Instead, now it actually wants to consume much more REM sleep and it's lost its appetite for deep non-REM sleep. So, in the second half of the night, the majority of those 90-minute cycles are comprised much more of REM sleep.