About the Online Alarm Clock
This online alarm clock rings in your browser at the time you choose. Set the alarm time, give it a label, pick a sound and press Set alarm — a live clock and countdown show exactly how long is left. When it goes off you can stop it or snooze for a few minutes. Nothing to install and no sign-up.
It is handy for power naps, reminders while you work at your computer, cooking and study sessions, or as a backup wake-up alarm when your phone is charging in another room. Keep this tab open and your volume up: browsers only play sound from open tabs, and some mobile browsers pause pages that are in the background or when the screen locks.
Below the alarm, the planner shows how long you have until it rings and how many 90-minute sleep cycles fit in that time, so you can pick a wake-up time that lands at the end of a cycle rather than in deep sleep.
How to use the online alarm clock
- 1Enter the time you want the alarm to ring, or tap a quick-set button such as +10 min.
- 2Add a label and choose an alarm sound, then press Test to check your volume.
- 3Press Set alarm and keep this tab open with sound on.
- 4When it rings, press Stop, or Snooze for a few more minutes.
- 5Use the sleep-cycle table to pick a wake-up time that ends a full cycle.
Formula and method
If the alarm time is earlier than the current time, it is set for the next day, so 24 hours are added before taking the remainder. An alarm set for exactly the current time rings in 24 hours.
The sleep planner subtracts the minutes it usually takes you to fall asleep and divides by 90 minutes, the average length of one sleep cycle. Waking near the end of a cycle, during lighter sleep, tends to feel less groggy. Real cycles range from about 80 to 120 minutes, so treat the times as a guide.
- Now
- Current time or the time you go to bed
- Alarm
- Time the alarm is set to ring
- 90 min
- Average length of one sleep cycle
Worked examples
Bed at 10:30 PM, alarm at 6:30 AM
From 22:30 to 06:30 is 8 hours. Allowing 15 minutes to fall asleep leaves 465 minutes, which is 5.17 ninety-minute cycles. Five complete cycles end at 6:15 AM.
20-minute power nap
The alarm rings 20 minutes after 1:00 PM. A short nap of 10–20 minutes keeps you out of deep sleep, so you wake up without the grogginess of a full cycle.
Late night: 11:45 PM to 7:00 AM
The alarm is 7 hours 15 minutes away. After 15 minutes to fall asleep you have 420 minutes, or 4.67 cycles; four full cycles would end at 6:00 AM.
Frequently asked questions
Will the online alarm ring if my computer is asleep?+
No. The alarm runs inside this browser tab, so the tab must stay open and the device awake. Plug in a laptop and disable sleep mode for long alarms, and keep the volume turned up.
Does the alarm work on my phone?+
It works while the page is open and on screen. Many mobile browsers pause background tabs and block sound when the screen locks, so for overnight alarms on a phone the built-in clock app is more reliable.
How long is a sleep cycle?+
A full cycle through light, deep and REM sleep lasts about 90 minutes on average, typically between 80 and 120 minutes. Most adults go through four to six cycles a night.
How much sleep do adults need?+
The American Academy of Sleep Medicine and the CDC recommend at least 7 hours per night for adults aged 18–60. Teenagers need 8–10 hours and school-age children 9–12 hours.
How long should a power nap be?+
Around 10 to 20 minutes. That is long enough to feel refreshed but short enough to avoid deep sleep, which can leave you groggy when you wake.
Sources