Lull: Sleep Cycle Calculator

Slept a full night and still woke up groggy? Part of the reason can be when the alarm goes off, not just how long you slept. Wake in the middle of deep sleep and getting up can feel like a struggle. Wake at the end of a sleep cycle and it tends to feel easier for many people.

Lull does the sleep math for you. Tell it when you need to be up and it shows the bedtimes that line up with full cycles, so there is no counting back from your alarm at midnight. Each cycle is about 90 minutes in adults (a little shorter or longer with age).

For example, to be up at 6:30 am, the adult setting suggests getting into bed at 10:45 pm for five full cycles (7h 30m). Want a shorter or longer night? Tap another option to compare.

Three ways to use it
- Wake up: enter the time you need to be up and get bedtimes counted back in full cycles.
- Go to bed: enter when you're heading to bed (or tap "use current time") and get wake-up times counted forward.
- Nap: choose a start time and a goal - a 20-minute power nap, a 30-minute nap, a full 90-minute cycle or a coffee nap - and see when to wake.

Every result shows the time, the number of cycles and the total sleep, and each estimate allows about 15 minutes to fall asleep.

Simple by design
- Quick: tap the time to pick it with a wheel, or nudge it in 15-minute steps
- Fits you: age presets (6-12, 13-19, 20-54 and 55+) adjust the cycle length
- Clear: a short "why these times" explainer, plus links to our sleep guides at instacalc.co
- Easy on the eyes: a dark, low-glare design for use in bed
- Private: no account or sign-up. The calculation happens on your device, and Lull doesn't store or send the times you enter
- Optional vibration feedback, with an off switch in the About screen

What Lull is not
Lull is a timing calculator. It doesn't set alarms, track your sleep or help you fall asleep faster, so use your phone's clock app for the alarm itself. Cycle length varies from person to person and night to night, so treat the times as a starting point and adjust to how you feel. This is an educational estimate, not medical advice. If you have ongoing trouble sleeping, please talk to a doctor.

Free, with one small banner ad
Lull is free to use and shows a single banner ad at the bottom of the screen. There are no full-screen ads. A one-time "Remove Ads" purchase in the About screen switches the banner off, and "Restore purchases" is in the same place. Where the law requires it, the app asks for your ad consent first, and you can change your choice later under "Ad privacy options". See the Data safety section for what the ad and crash-reporting services collect.

Lull is the app companion to the sleep calculator and guides at instacalc.co.

Built by Quiet Hours Studio.
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