Privacy Policy
Last updated: 8 September 2026
KnowSleep ("the app") is developed and operated by Beijing Zhang Liang World Technology Co., Ltd. This policy explains how the app handles your information.
The conclusion first: the app has no server, no account system, and collects, uploads and stores no personal information whatsoever. Everything below is that sentence in detail.
1. What we collect
Nothing. The app has no backend, so "data on our side" does not technically exist. Specifically, the app:
- requires no registration — no email address, phone number, or third-party sign-in;
- collects no name, gender, date of birth, location, contacts, photos, microphone or camera data;
- collects no device identifiers (IDFA, IDFV) and performs no device fingerprinting;
- integrates no advertising SDKs and no third-party analytics SDKs;
- sends no crash logs to any server of ours. (If you have opted in to sharing with developers in your system settings, Apple may give us anonymous aggregate crash statistics. These contain none of your health data; see Apple's privacy policy.)
2. How health data is handled
The app's core feature reads data from Apple Health. That data is read and processed only on your iPhone and never leaves your device.
With your explicit permission, the app reads the following types: sleep stages, heart rate, resting heart rate, heart rate variability (HRV), blood oxygen saturation, respiratory rate, wrist skin temperature, step count, workouts, and caffeine intake; on iOS 18 and later it also reads sleep apnea events. Resting heart rate and step count are currently read and stored only — nothing is concluded from them yet.
- There is exactly one thing it can write, and it is off by default: if you turn on 「Sync Caffeine to Health」 in the app's Settings, the coffee you tag in the app is written back to Apple Health as caffeine intake. Nothing else is ever written. You can turn the switch off at any time; writing then stops, and anything already written can be deleted from the Health app yourself;
- all analysis, comparison and conclusions happen on the device, with no network involved;
- permission is granular — you may grant only some types, and you may revoke it at any time under Settings → Health → Data Access & Devices on your iPhone;
- after revoking, the app loses access and the related features stop working. No residual copy is left behind, because none was ever made.
3. The behaviour tags you record
The behaviour tags you tap in the app (coffee, alcohol, exercise, late dinner, a heavy dinner, phone use before bed, stress, illness, a daytime nap, a change of routine, a warm bedroom), together with anything you add in a daily reading, are stored in the app's sandbox on your device, protected by your iPhone's system encryption. They are never uploaded, never synced, and we cannot read them.
4. Where data lives, and for how long
All data — your tags, the conclusions computed locally, and your preferences — lives in the app's sandbox and is accessible only to the app. How long it is kept is entirely your decision: you can wipe it under Settings → Your Data (「Delete every reading and answer」 and 「Clear All Tags」), or simply delete the app, in which case the system removes it with the app. Deletion is irreversible, because we hold no backup.
5. What we do not do
- We do not sell, rent or trade any of your data;
- we do not use health data for advertising, marketing or profiling;
- we do not share health data with any third party;
- we do not use your data to train any model.
6. Third parties
The app integrates no third-party SDKs. It uses only Apple's own platform services:
- HealthKit — reads the health data listed above, entirely on-device;
- StoreKit — handles in-app purchases and subscriptions. Payment is handled by Apple; we never see your payment method, card number or billing address. All we learn from Apple is whether this device holds a valid purchase;
- Local notifications — if you turn on the evening reminder, your device schedules and fires it itself. No push server is involved.
Apple's handling of data in these services is governed by Apple's own privacy policy.
7. Children
The app is not directed at children under 13 and does not knowingly collect information from them. As the app collects no information at all, there is no children's data to delete.
8. Your rights
Under the GDPR, the CCPA, China's Personal Information Protection Law and other applicable laws, you have rights to access, port, correct and delete your personal information, and to withdraw consent. Because the app collects no personal information, exercising these rights is direct and immediate:
- Access and portability: Settings → Your Data → Export everything produces a JSON file containing your tags and every night's conclusions;
- Erasure: Settings → Your Data (「Delete every reading and answer」, 「Clear All Tags」), or delete the app;
- Withdrawing consent: revoke health permissions at any time under Settings → Health on your iPhone.
All of this happens on your device. There is no request to file with us, and no way for us to perform it on your behalf.
9. Security
Data never leaving the device is itself the strongest protection available: there is no transmission to intercept and no server to breach. On-device data is protected by the iOS sandbox and system encryption. Setting a passcode on your iPhone is the single most effective thing you can do to protect it.
10. Changes to this policy
If this policy changes, we will update the date at the top of this page. If a change materially affects how data is handled, we will say so clearly inside the app.
11. Health notice
The app does not diagnose. It is not a medical device and does not replace the professional judgement of a doctor or a medical service. Every conclusion it offers is derived solely from your own history and is a lifestyle observation, nothing more. For any question about your health, talk to a doctor.
12. Contact
For any question about this policy:
- Email: service@worldmetric.cn
- Operator: Beijing Zhang Liang World Technology Co., Ltd.