How sleep is structured
Adult sleep cycles repeatedly through stages of non-REM (light, deep) and REM sleep, with a typical cycle length of about 90 minutes. The first cycles of the night are dominated by deep sleep; later cycles contain proportionally more REM. Waking up at the end of a cycle, when you’re briefly closer to consciousness, is associated with feeling more alert than waking mid-deep-sleep.
Why we add a sleep-latency buffer
The cycle math only works once you’re actually asleep. Average sleep latency for a healthy adult is around 14 minutes — we add that when computing a wake-up time from a bedtime, so a “5-cycle” plan really gives you 7.5 hours of sleep, not 7.5 hours of lying in bed.
What the suggestion doesn’t guarantee
90 minutes is a population average. Your individual cycles might be 70 or 110. Stress, alcohol, late caffeine, irregular shifts and screens-in-bed all pull the actual cycles around. The bigger lever for sleep quality is consistency — same bedtime and wake-up time every day, including weekends — not which specific 90-minute multiple you target.