Wake between cycles, not mid-cycle. Get the best bedtimes or wake-up times based on 90-minute sleep cycles.
Sleep runs in cycles of roughly 90 minutes, moving from light sleep into deep sleep and then REM before starting again. Waking at the end of a cycle, in light sleep, feels far more refreshing than being jolted awake mid-deep-sleep, which causes grogginess. That is why timing sleep in whole cycles helps: the calculator above works backwards from your wake time (or forwards from now) to suggest bedtimes that land you at the end of a cycle.
How much you need. Most adults need seven to nine hours, which is about five to six full cycles; teenagers need more, and needs shift with age. Consistently getting less builds a sleep debt that harms focus, mood and health, and you cannot fully repay it with one long lie-in.
Make the cycles count. Timing helps, but sleep quality matters more than clever maths. Keep a regular schedule even at weekends, get daylight in the morning, avoid caffeine late in the day, and keep the bedroom dark and cool. Add about 15 minutes to fall asleep when planning your bedtime.