Is six hours of sleep enough if I complete four cycles?

Six hours is generally not enough sleep for most adults, even if it appears to equal four 90-minute cycles. Sleep cycles are not always 90 minutes, and completing a whole number does not remove the health effects of insufficient total sleep. Most adults need at least seven hours, with many requiring eight or nine. Restricting sleep to six hours can reduce later-night REM sleep and may lead to accumulating sleep debt. You might initially feel functional, especially with caffeine, but concentration, reaction time and mood can still decline. A small minority of people naturally require less sleep, but this is uncommon. Judge your needs by sustained daytime alertness rather than cycle arithmetic. Regularly needing catch-up sleep suggests six hours is insufficient.