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At the time you set, it runs a real shutdown command on your phone. It's the same kind of clean power-off you'd get from holding the power button and tapping "Power off". It is not a screen lock, not Do Not Disturb, and not grayscale. The device genuinely turns off.

Android reserves the shutdown API for system-signed apps (the OS, the phone manufacturer). No regular app permission unlocks it. Shizuku gives Nightcap the same access level that ADB has, which is enough to run reboot -p. That is the only way for a non-root, non-system app to power the phone off cleanly.

Two work with Nightcap:

• Official (RikkaApps/Shizuku, Play Store): stable and well-known. The trade-off is that on a non-rooted phone you must manually start it again after every reboot, so Nightcap needs a 5-second daily ritual.
• Community fork (thedjchi/Shizuku, GitHub): same core, plus a robust "start on boot" mode and a watchdog that restarts Shizuku if it crashes. With start-on-boot enabled, Shizuku reconnects automatically after the phone powers on, so Nightcap works without any manual restart step. The trade-off is that it asks for WRITE_SECURE_SETTINGS so it can toggle wireless debugging itself.

Both are free and open-source. Pick whichever you're comfortable with.

It calls the OS's own orderly shutdown (reboot -p), which is exactly what tapping "Power off" in the menu does. Storage unmounts cleanly, so the risk is no different from powering the phone off yourself from the menu. Nightcap doesn't modify any system files. It also has guardrails: no shutdown if notifications are off, no shutdown without a valid scheduled trigger, and a 5-minute grace window so a badly-delayed alarm can't surprise-kill the phone later.

No. Shizuku runs at ADB-shell privilege, which on most consumer Android phones is enough to call reboot -p. Root works too, but it's not required.

Only if you're on the official Shizuku. The official build doesn't carry the WRITE_SECURE_SETTINGS permission, so on a non-rooted phone it can't re-enable wireless debugging and start itself after a reboot; you have to do that manually. It's a Shizuku limitation, not a Nightcap one. The community fork (linked above) holds WRITE_SECURE_SETTINGS and has a "start on boot" toggle that solves this.

No. Modern Android removed scheduled power-on across manufacturers, and Nightcap can't bring a powered-off phone back. You'll need to power it on manually when you want to use it again. This is by design: the whole point is that you have to deliberately turn it back on.

No. A powered-off phone cannot ring alarms. If you rely on the phone for any alarm during the time it'll be off, either set your Nightcap cutoff after those alarms would fire (for example, schedule cutoff for noon if your wake alarm is at 7 am), or use a separate alarm clock to cover the off window.

Yes. Each cutoff (by default) shows a live countdown notification with a Cancel button starting 60 seconds (or whatever length you chose) before the cutoff. Tap Cancel and the shutdown is aborted. You can also flip the master switch in Settings.

Hardcore mode. Once you save a cutoff with Uninterruptible on, you can no longer turn that cutoff off from Home, and the Cancel button on its countdown notification is forced off. The only way to stop a fired countdown is the master kill-switch in Settings. You can still delete an Uninterruptible cutoff from the editor. Useful when you know you'll talk yourself out of stopping at the time you set, but think before enabling.

Three usual suspects: (1) Shizuku isn't running; open it and check it's started. (2) The OS killed the app in the background; open Nightcap, go to Settings, then Cutoffs not working? and walk through the per-OEM steps. (3) The master kill-switch in Settings is off. The Home screen's Setup checklist flags most of these.

No. The app collects nothing about you and sends nothing to us; we don't run any server. The only network use is Google Play Billing if you tap to buy the optional Pro upgrade. That talks only to Google's servers and is governed by Google's Privacy Policy. Your cutoffs, history, and settings stay in the app's private storage on your device. See the privacy policy for the full details.