Your day, coming around.
Precise inputs → measured outputs → audited insights → a live forecast for tonight.
Circa pairs disciplined lifestyle logging with your Fitbit Air's physiology —
computed entirely on your iPhone.
Circa is a free, private iPhone app that pairs disciplined lifestyle logging — supplements, caffeine, and nicotine — with your Fitbit Air's physiology. It computes audited correlations with n, a 95% confidence interval, and p, plus a transparent nightly sleep forecast, entirely on your iPhone. No account, no servers, no analytics.
Download on the App Store — freeThe app
— Today —
— Log —
— Signals —
What it does
Stimulant pharmacokinetics
Live caffeine and nicotine decay curves against your bedtime — with the 50 mg threshold drawn where it matters.
Audited insights
Every effect ships with n, 95% CI, p — and the actual nights behind it. Anything failing its interval for 14 days is retired automatically.
Tonight forecast
A transparent driver model that moves as you log. Every driver shows its signed contribution — no black box.
104-supplement library
Goal-ranked, evidence-graded, interaction-flagged. One tap from the library to your stack.
Where your data lives
On your iPhone. Nowhere else. Health data is fetched read-only from the Google Health API and processed locally. No servers, no analytics, no accounts, no sale of data. Disconnecting revokes the token and erases everything.
Questions
Is Circa free?
Yes. Circa is free to download on the App Store, with no account required. It pairs your lifestyle logging with your Fitbit Air's physiology to surface audited insights and a nightly sleep forecast, all computed on your iPhone. There are no servers, no analytics, and no sale of your data.
What does Circa track?
Circa logs your daily supplements, caffeine, and nicotine, then pairs that lifestyle data with your Fitbit Air's physiology. It draws live caffeine and nicotine decay curves against your bedtime, with the 50 mg threshold marked, and ranks a 104-supplement library by goal, evidence grade, and interaction flags.
How does Circa's sleep forecast work?
Circa computes a transparent nightly sleep forecast from a driver model that updates as you log. Every driver shows its signed contribution, so there is no black box. The forecast re-fits as more nights accumulate, and insights that fail their confidence interval for 14 days are retired automatically.
Is my health data private in Circa?
Yes. Circa processes everything on your iPhone and nowhere else. Health data is fetched read-only from the Google Health API and computed locally — there are no servers, no analytics, no accounts, and no sale of data. Disconnecting revokes the access token and erases everything on your device.
What are audited insights in Circa?
Audited insights are correlations that Circa reports honestly. Every effect ships with its sample size (n), a 95% confidence interval, and a p-value, along with the actual nights behind it. Any insight that fails its interval for 14 days is retired automatically, so weak findings do not linger.
Which device and iPhone does Circa need?
Circa is an iPhone app for iOS 17 and later, and it reads physiology from the Google Fitbit Air. You log supplements, caffeine, and nicotine yourself, and Circa pairs that with the Air's data. Home Screen and Lock Screen widgets let you log an intake with a single tap.
Is Circa a medical device?
No. Circa is a personal tracking tool, not a medical device, and it does not diagnose, treat, or prevent any condition. It presents evidence grades and audited correlations for your own information, not prescriptions. Always consult a qualified professional before making health or supplementation decisions.