Sleep calculator
Sleep cycles run roughly 70 to 120 minutes and lengthen through the night as REM takes a larger share. So the tidy arithmetic of five 90-minute cycles is an approximation rather than a schedule. What is well supported is the underlying idea: waking at the end of a cycle, in light sleep, feels far better than being pulled out of deep sleep partway through one.
General guidance. Persistent poor sleep, loud snoring or daytime sleepiness are worth discussing with a doctor rather than solving with arithmetic.
Sleep runs in cycles of roughly 90 minutes, and waking at the end of one feels better than waking mid-cycle. Five cycles is 7.5 hours asleep, or about 7 hours 44 minutes in bed once you allow the average 14 minutes to fall asleep.
How to use sleep cycles
Total sleep matters far more than cycle alignment. Six cycles that end at an awkward moment beat four that end perfectly, and the research on sleep duration is much stronger than the research on wake timing. Cycle counting is most useful when sleep is going to be short anyway: given a choice between six hours and six and a half, ending at a cycle boundary is a reasonable tiebreaker. It is not a reason to sleep less.
Questions
About 90 minutes on average, but genuinely ranging from 70 to 120 and lengthening through the night.