The Best Free Circadian Rhythm Tools That Work in Your Browser
Circadian rhythm tools have a reputation for being expensive. The most-cited products — Rise Science ($70/year), WHOOP ($239/year), Oura Ring ($299+) — carry real subscription costs before you know whether the approach actually changes how you work.
There are genuinely useful free options. This roundup covers the best ones — prioritizing tools that work in the browser, require no account, and deliver immediate value without commitment.
1. Circadianly (Browser Extension) — Best for ambient zone awareness
Free. No account. No data collection. Works offline.
Circadianly is a Chrome extension that shows your current circadian energy zone in the browser toolbar while you work. You enter your wake time once; it calculates your five zone windows (Morning Activation, Cognitive Peak, Afternoon Dip, Creative Rebound, Evening Wind-Down) and tracks which zone you’re currently in throughout the day.
The entire calculation runs locally in your browser using chronobiological zone offsets derived from your wake time — no server, no account, no data sharing. It optionally integrates with Google Calendar to show your events alongside zone information, but this is local to your browser session and doesn’t transmit calendar data anywhere.
Best for: Knowledge workers who spend their day in a browser and want zone awareness as ambient context rather than a notification or app they have to check.
Limitation: Doesn’t model sleep debt. If you’re working on 4 hours of sleep, it doesn’t know — it shows you the same zone windows it would on a well-rested day.
2. Circadianly Energy Zone Calculator (Web Tool) — Best for one-off zone lookup
Free. No install. No account.
The energy zone calculator on this site lets you enter a wake time and instantly see all five zone windows for the day, including which zone you’re currently in (highlighted). No installation required — useful if you want to check your schedule for a specific day without committing to an extension.
Best for: Quickly checking your zone schedule before an important day, or evaluating whether the circadian zone model is useful before installing anything.
3. Cognitive Peak Calculator — Best single-metric lookup
Free. No install. No account.
The cognitive peak calculator gives you the one number most people actually need: when is your brain at its best today? Enter your wake time; it returns your peak window (wake + 2h to wake + 6h) with a note if you’re currently in it.
Best for: People who don’t need the full five-zone picture and just want to know when to schedule their hardest work.
4. Rise Science (Free Trial) — Best if you want sleep debt modeling
Free trial available; $70/year after.
Rise Science offers a free trial that shows their full energy curve including sleep debt adjustment. If you want to see what sleep-debt-aware energy modeling looks like before committing, the trial is worth trying. After the trial, it’s a paid subscription — but the trial period is genuinely useful for comparison.
Best for: Users who want to evaluate whether sleep debt modeling is worth paying for, particularly those with variable sleep schedules.
Limitation: Requires account creation and phone data access. After the trial, costs $70/year.
5. Sleep Foundation Sleep Hygiene Tools — Best for understanding the science
Free. Educational.
The Sleep Foundation provides research-backed information about circadian rhythm timing, chronotype assessment, and sleep optimization. Not a tracking tool, but a reliable reference for understanding the science behind zone-based scheduling.
Best for: Understanding the research before choosing a tool, or explaining the biology to a skeptical colleague.
How to choose
| Need | Best free option |
|---|---|
| Zone awareness while working at computer | Circadianly extension |
| Quick zone lookup, no install | Energy Zone Calculator |
| Single number: when is my peak today | Cognitive Peak Calculator |
| Sleep debt modeling (trial) | Rise Science (free trial) |
| Understanding the science | Sleep Foundation resources |
What the free tools don’t cover
The free tools listed here all use wake-time-based circadian modeling. They don’t track sleep debt (how rested you actually are), physiological state (HRV, heart rate), or longitudinal recovery trends. If those factors matter to your use case, a wearable or paid subscription may be worth the investment.
For most knowledge workers deciding when to schedule deep work versus administrative tasks — the most common use case for circadian awareness — wake-time-based tools cover the use case fully. See Do You Need a Wearable to Track Your Energy? for a fuller comparison.
The key question is whether you’ll actually change scheduling decisions based on this information. If the answer is yes, start with the free tools. If you find yourself wanting more precision after a month of use, the paid options are justified upgrades.
Related: The 5 Circadian Energy Zones · Circadianly vs Rise Science · Do You Need a Wearable?