Placebo effect. Sleep doesn't work in neat 90 minute cycles. They're really longer in the early part of the night, and get faster and more biased towards REM i

Placebo effect. Sleep doesn't work in neat 90 minute cycles. They're really longer in the early part of the night, and get faster and more biased towards REM in the later part of the night. The details are very individual and also vary night to night, so you can't predict with any accuracy at the start of the night what stage you'll be in at 7:00:00AM the next morning. (Anecdote: A friend of mine used to swear by a theory, mostly as a joke, that the recipe for good night's sleep was sleeping an odd number of hours. One day he came all happy and said, "Thank God 15 is an odd number!")