iPhone · iOS 17 and later · free
Caffeine, Nicotine and Supplement Tracker for iPhone
Circa logs what you take down to the exact dose and minute, reads your tracker's sleep and HRV from the Google Health API, and forecasts tonight's deep sleep from the two together. Everything is computed on your iPhone.
Free · no account · ad supported · no in-app purchases
The four tabs
What the app actually looks like
These are the real App Store screenshots, shown in demo mode.
What it does
Four things, done precisely
Stimulant curves that end at your bedtime
Caffeine and nicotine decay curves run from your first log to the moment you go to bed, with a 50 mg caffeine line drawn across them. You see what this afternoon's espresso is still worth at 23:00 instead of guessing.
Audited insights, with the receipts attached
Every effect carries n, a 95 percent confidence interval, a p-value, and the nights behind it. An effect whose interval fails to clear zero for 14 days straight is retired automatically. Nothing is reported as a finding until it has earned the word.
A forecast that shows its work
A linear driver model predicts tonight's sleep score and deep-sleep minutes. Caffeine at bedtime, nicotine after 20:00, your evening stack, zone minutes and sleep debt each show a signed contribution. It re-fits to your own nights by ridge regression and recomputes on every log.
A 104-entry supplement library
Each entry carries a category, a benefit, a typical dose, a timing note, an interaction note and an evidence grade, and the list re-ranks itself around the goals you pick. It is reference material compiled from the NIH databases and Examine, not a recommendation.
Sources
Exactly what Circa reads
Circa does not use Apple HealthKit and it does not talk to the Fitbit Web API. It reads the Google Health API directly, over four read-only scopes, and writes nothing back.
You need a tracker whose data reaches your Google account, plus a Google account, for the physiology half. Manual logging, the stimulant curves, the supplement library and demo mode all work with no account at all.
Who it is for
Three questions it was built to answer
"Is my afternoon coffee costing me deep sleep?"
Log the espresso, watch the curve, and after enough nights read the fitted effect: deep-sleep minutes per 100 mg of caffeine still on board at bedtime, controlled for training load, weekday, alcohol and bedtime.
"Does the pouch before bed actually wreck my REM?"
Circa splits your nights into those with nicotine logged after 20:00 and those without, compares measured REM minutes between the two groups, and reports the difference with a confidence interval rather than a verdict.
"Is this supplement doing anything for me?"
Run it as an N of 1 A/B trial. Circa tracks the on weeks against the off weeks, shows the running effect and p-value, and tells you which week the dose-response readout unlocks.
Where your data lives
On your iPhone. The ads are the exception, and they never see it.
No Circa account, no Circa server
There is nothing to sign up for and nothing to sign in to. Health data read from the Google Health API is stored in a local database on your phone and every statistic is computed there.
Advertising is the one network dependency
Circa is free because it shows ads supplied by Google AdMob, and those are fetched over the network. They are capped hard: none during your first few actions, at most one full-screen ad per session, and never closer than three minutes apart. Your logs and your health data are never sent to them.
Disconnect erases everything
Disconnect and erase revokes Circa's Google token and wipes the local database on the spot. Deleting the app does the same. Before that, you can export a CSV of every intake plus a full JSON dump through the share sheet.
Circa is a personal tracking tool, not a medical device. It does not diagnose, treat, or prevent any condition, and nothing in the app is medical advice. Read the full privacy policy.
Questions
Before you download
Is Circa really free?
Yes. Circa costs nothing to download and there are no in-app purchases and no subscription. It pays for itself with advertising supplied by Google AdMob, which is loaded over the network. Ads are capped: at most one full-screen ad per session, never in the first few actions, and never closer than three minutes apart. There is no paid tier that removes them.
What does Circa read, and from where?
Two things. You log supplements, caffeine and nicotine yourself, with an exact dose, an exact unit and an exact time. Circa reads physiology from the Google Health API over four read-only scopes: steps and active zone minutes, resting heart rate, heart rate variability, blood oxygen, breathing rate, VO2 max, distance, active energy, intraday heart rate, and sleep sessions. Circa does not use Apple HealthKit and does not talk to the Fitbit Web API.
Do I need a Fitbit to use it?
For the physiology, yes: you need a tracker whose data lands in your Google account, and a Google account to authorise the read. Manual logging, the caffeine and nicotine curves, the supplement library and demo mode all work without connecting anything.
How does the sleep forecast work?
A linear driver model runs on your iPhone. Caffeine still in your system at bedtime, nicotine logged after 20:00, your evening stack, active zone minutes and sleep debt each contribute a signed number of deep-sleep minutes, and every contribution is shown. It starts from published starting coefficients and re-fits to your own nights by ridge regression once you have around 30 of them. It recomputes on every log.
What makes an insight audited?
Every effect carries its receipts: the sample size n, a 95% confidence interval, a p-value, and the individual nights behind it. An effect only counts when its interval stays clear of zero. If it fails that test for 14 consecutive days it is retired automatically. Insights also need a minimum number of nights and at least 80 percent log completeness in their window before they unlock. Null results are reported as null results.
Where does my health data go?
Nowhere. Circa has no account and no server of its own, and the health data it reads is stored and computed on your iPhone. Disconnect and erase revokes the Google token and wipes the local database. You can export everything at any time as a CSV of your intakes plus a full JSON dump. The advertising is the one thing that uses the network, and it never receives your health data.
What does Circa deliberately not do?
There is no barcode scanner, no branded-drink database, no reminders or notifications, no Apple Watch app, no iCloud sync, and no coaching. It logs precisely, reads your tracker, and reports statistics honestly. Anything else is out of scope on purpose.
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