Sundial: Body Clock
Set your body clock with light
Only for iPhone
Free · In‑App Purchases · Designed for iPhone. Not verified for macOS.
iPhone
Trips, shift changes, weekend sleep-ins: one body-clock problem. Sundial reads your needle from Apple Watch and works only on the gap. Aligned days stay silent.
Your body clock has a needle: the time your core temperature is lowest. Light before it delays the clock; light after it advances the clock. So "get morning sun" is right at home on your usual schedule, and backwards on the first morning in a new time zone or after a weekend sleep-in.
Sundial estimates where that needle is from your Apple Watch sleep and heart rate during sleep, and looks only at the gap to where you want it.
■ Three situations, one mechanism
• Trips: shift your sleep step by step for a few days before departure; after arrival, get windows for seeking and avoiding light in local time. Eastward and westward are different jobs, and it knows.
• Shift changes: from the last day of your old shift, use your days off to move gradually. Enter working hours and it can start on work days too.
• Daily life: only on a morning your needle has drifted (say, after a weekend), you get a timetable to bring it back. On aligned days: "Today is free."
■ A five-line timetable
Seek/avoid light, wake and bedtime, first and last bite, exercise window, bath. All computed from your needle for that day only.
■ No assumptions
It doesn't assume you wake at 7. Your baseline is learned from your own last 14 wake times. Night owls and late shifts keep their own life as the baseline.
■ Your data stays on your device
Apple Health data and everything computed from it stay on your iPhone. No server. No ads, no tracking. Every feature is free for individuals.
■ Evidence
Needle = core temperature trough, 3–4.5 h before waking (Eastman & Burgess 2009). Light phase response (Khalsa 2003), exercise phase response (Youngstedt 2019), meals and peripheral clocks (Wehrens 2017), pre-flight adjustment (Burgess 2003). Sundial is not a medical device and does not promise to cure jet lag.
Apple Watch (sleep tracking) recommended. Without a Watch, enter your usual wake time and the timetable still works.
Ratings & Reviews
- This app hasn’t received enough ratings or reviews to display an overview.
- Tomorrow's notifications are booked the day before, so they arrive even if the app is closed
- Fixed wake targets sometimes moving away from the desired time
- Fixed tomorrow's plan disagreeing with the schedule in Plans
- Settings now save when the sheet is swiped down
- Fixed city names being read as airport codes when importing trips
- English, Dynamic Type and VoiceOver improvements
The developer, Ryosuke Hoshino, indicated that the app’s privacy practices may include handling of data as described below. For more information, see the developer’s privacy policy .
Data Not Linked to You
The following data may be collected but it is not linked to your identity:
- Health & Fitness
- Identifiers
Accessibility
The developer has not yet indicated which accessibility features this app supports. Learn More
Information
- Seller
- Ryosuke Hoshino
- Size
- 4.3 MB
- Category
- Health & Fitness
- Compatibility
Requires iOS 18.0 or later.
- iPhone
Requires iOS 18.0 or later. - Mac
Requires macOS 15.0 or later and a Mac with Apple M1 chip or later. - Apple Vision
Requires visionOS 2.0 or later.
- iPhone
- Languages
- English and Japanese
- Age Rating
13+
- 13+
- Infrequent
Medical Treatment information
- In-App Purchases
Yes
- Generous $9.99
- A lunch $4.99
- A coffee $1.99
- Copyright
- © 2026 Lei lab