SunCycle

About SunCycle

SunCycle is a free, static, science-backed circadian light scheduler. Built on real solar geometry and photobiology. No paid APIs, no tracking, no accounts.

Philosophy

Sleep is not a luxury — it is a biological necessity. Yet modern life has decoupled us from the light-dark cycle that evolved to regulate our circadian rhythms. SunCycle exists to bridge that gap: a simple, free tool that translates real solar data into actionable light guidance.

We believe that the best health tools are private by default, science-backed, and accessible to everyone. No paywalls. No data mining. Just physics, physiology, and a clean interface.

How it works

SunCycle computes your local solar events using the NOAA solar position algorithm adapted for browser JavaScript. It calculates sunrise, sunset, solar noon, twilight windows, and golden/blue hours from your latitude, longitude, and IANA timezone.

Then it maps those solar events to peer-reviewed light-exposure recommendations: morning bright light, midday daylight maintenance, evening dim-down, and a blue-light cutoff before sleep. Your chronotype shifts the windows to match your biology, not an arbitrary clock.

Astro 5 TypeScript Vanilla CSS NOAA Solar Algorithm CIE S 026:2018

Privacy

No data leaves your browser. SunCycle is a static site. Geolocation, if you grant it, is handled entirely by your browser. We do not run a server, do not store your location, and do not use cookies or analytics.

Sharing a plan creates a URL with lat/lon/tz in the query string. Anyone with the link can see your coordinates. Do not share if that concerns you.

Sources

  • Brainard et al. (2001, 2015) — ipRGC spectral sensitivity and melanopic lux.
  • CIE S 026:2018 — melanopic Equivalent Daylight Illuminance (mel-EDI).
  • Figueiro et al. (2017) — morning light for circadian phase advancement.
  • Zeitzer et al. (2000) — evening light avoidance and phase response.
  • Provencio et al. (2002) — melanopsin discovery in ipRGCs.
  • Pauley et al. (2004) — daylight exposure and sleep quality.

See The Science for a deeper dive into each topic and a research timeline.

Cities in the directory

50 cities with known lat/lon/timezone. If your city is missing, use Use my location or pick the nearest match.

  • New York, US
  • Los Angeles, US
  • Chicago, US
  • Houston, US
  • Phoenix, US
  • Seattle, US
  • San Francisco, US
  • Philadelphia, US
  • Boston, US
  • San Diego, US
  • Denver, US
  • Miami, US
  • London, UK
  • Paris, FR
  • Berlin, DE
  • Rome, IT
  • Madrid, ES
  • Tokyo, JP
  • Seoul, KR
  • Beijing, CN
  • Shanghai, CN
  • Hong Kong, HK
  • Singapore, SG
  • Mumbai, IN
  • New Delhi, IN
  • Bangkok, TH
  • Sydney, AU
  • Melbourne, AU
  • Perth, AU
  • Rio de Janeiro, BR
  • São Paulo, BR
  • Buenos Aires, AR
  • Mexico City, MX
  • Toronto, CA
  • Montreal, CA
  • Vancouver, CA
  • Copenhagen, DK
  • Stockholm, SE
  • Helsinki, FI
  • Oslo, NO
  • Reykjavík, IS
  • Dubai, AE
  • Johannesburg, ZA
  • Cape Town, ZA
  • Cairo, EG
  • Amsterdam, NL
  • Vienna, AT
  • Prague, CZ
  • Budapest, HU
  • Istanbul, TR

Limitations

  • The solar algorithm is accurate to ~1 minute for non-polar latitudes. Polar regions near solstice may see larger deviations.
  • Light prescriptions are population-level guidance, not personalized medical advice.
  • Weather is not modeled. Overcast outdoor light is still 2,000–10,000 lux — sufficient for entrainment.
  • Shift work is not supported in v1.0. The planner anchors to local sunrise and sunset.
  • Chronotype quiz is a simplified 5-question screen, not a full MCTQ assessment.