How Sleep Cycles Work
Sleep occurs in cycles of roughly 90 minutes, each consisting of light sleep, deep sleep, and REM (rapid eye movement) sleep. Waking in the middle of a cycle—especially during deep sleep—can leave you feeling groggy and disoriented. This Sleep Calculator helps you plan when to go to bed or when to set your alarm so that you wake between cycles, when your body is in a lighter stage of sleep and you are more likely to feel refreshed.
The calculator assumes it takes about 15 minutes to fall asleep, so it factors that into the timing. You can choose how many 90-minute cycles you want (typically 4–7, equating to 6–10.5 hours of sleep). Most adults function best with 7–9 hours, which corresponds to about 5–6 full cycles. If you need to wake at a specific time, the calculator tells you when to aim for bed; if you know when you'll go to sleep, it suggests ideal wake times.
Individual sleep needs vary, and this tool is a guide rather than a guarantee. Factors such as sleep quality, stress, and caffeine can affect how rested you feel. Prioritize consistent sleep and wake times, a dark and cool sleep environment, and limiting screens before bed. If you struggle with persistent fatigue or sleep problems, consider consulting a healthcare provider.