Set an alarm for 12:51 AM
A free online alarm that rings at 12:51AM. Start it, leave the tab open, and it will sound at 12:51 AM. No app, no account, nothing to install. The countdown below shows exactly how long is left.
What time should you go to bed to wake at 12:51AM?
Sleep runs in cycles of roughly 90 minutes, and waking at the end of a cycle feels easier than waking in the middle of one. Allowing about 14 minutes to fall asleep, these bedtimes line up with a 12:51 AM alarm:
| Go to bed at | Cycles | Sleep |
|---|---|---|
| 3:37 PM | 6 | 9.0 hours |
| 5:07 PM | 5 | 7.5 hours |
| 6:37 PM | 4 | 6.0 hours |
Cycle length varies between people, so treat these as a guide rather than a prescription.
Who sets a 12:51AM alarm
Midnight alarms are usually about a deadline, a release, or a billing cut-off rather than waking up. Because the alarm lands exactly as the calendar date changes, it is worth checking whether the thing you are timing uses your local midnight or UTC.
If you go to bed now, when should the alarm ring?
Read the table the other way round when the bedtime is fixed rather than the wake time. Starting from a 12:51 AM alarm, these are the sleep totals you get for each bedtime:
| Sleep wanted | Bedtime | Cycles |
|---|---|---|
| 9 hours | 3:37 PM | 6 |
| 7.5 hours | 5:07 PM | 5 |
| 6 hours | 6:37 PM | 4 |
| 4.5 hours | 8:07 PM | 3 |
How the alarm works
The timer reads your device clock, so 12:51 AM means 12:51 AM wherever you are, with no time zone to configure. Pressing start schedules a tone for the next 12:51 AM; if that time has already passed today, it is set for tomorrow. The sound is generated in the browser rather than loaded as a file, so it works offline once the page is open. The tab needs to stay open, and on phones the screen should stay awake, since browsers suspend background tabs.
Questions
- How do I set an alarm for 12:51AM?
- Press start above. The alarm sounds at 12:51 AM by your device clock.
- Will the alarm work if I close the tab?
- No. Browsers stop timers in closed tabs, so leave this page open.
- What is 12:51AM in 24-hour time?
- 12:51 AM is 00:51, or 0051 hours in military time.