Night clock

Is it bad to have a clock on all night?

A bright one, yes. Light in the evening and overnight is the thing most likely to interfere with falling asleep, and a phone at full brightness on a bedside table is a small floodlight. A dim red-shifted display avoids most of that, which is why this face is deliberately low contrast rather than the crisp white of the main clock.

The other half of the problem is checking it. Waking at 3am and calculating how little is left before the alarm reliably makes getting back to sleep harder. If that is a habit, turning the clock away from the bed is a better fix than any setting here.

How do I stop the screen turning off?

The page holds a wake lock while the night clock is open, so the display stays on without changing anything in your system settings. Put the device on a charger, since holding the screen awake all night will empty a phone battery by morning. Press F for full screen and the browser chrome goes too.

Will it work as a bedside clock on an old phone?

That is close to the ideal use for it. Nothing is downloaded, nothing runs on a server, and the page is light enough to sit open for hours on hardware too old for anything else. An old handset on a charger makes a better bedside clock than most devices sold as one, and it will not try to sell you anything at 4am.