Will my alarm go off in airplane mode?

Short answer: yes. On both iPhone and Android, an alarm set in the built-in Clock app rings in airplane mode, and so does almost every alarm app built for the job. Airplane mode switches off the phone's radios. It does not switch off its clock.

There are, though, three situations that really can leave you sleeping through a flight-mode alarm, and none of them is airplane mode itself. They are worth knowing before an early flight, a night train or a first morning abroad.

Why airplane mode does not matter

An alarm is not something the phone fetches from the internet. When you set one, the phone schedules it on the device itself, and at the right moment the phone wakes up and rings. No signal, no Wi-Fi and no server are involved.

That is also why an alarm rings in a hotel with Wi-Fi that never connects, in a basement with no reception, or on a plane at cruising altitude. The phone already knows what to do, and it does not need to ask anyone.

The three things that can actually catch you out

1. The phone is switched off. Airplane mode is fine; powered off is not. Don't count on any alarm ringing from a phone that is turned off, and keep an eye on the battery. A phone at 4% at midnight may not make it to 6:00, and an alarm that never rings because the phone died is indistinguishable from one that failed.

2. The alarm sound needs the internet. If your alarm plays a song from a streaming service, an online radio station or a playlist that isn't downloaded, the sound itself needs a connection. Some apps fall back to a plain tone when they can't reach it; some may not. The night before a trip, pick a sound that is stored on the phone.

3. The phone is still on the old time zone. Phones usually set their time zone from the mobile network or your location. If you have been in airplane mode since you landed, your phone may still think it is at home, so a 7:00 alarm rings at 7:00 home time. Look at the clock on your lock screen before you go to sleep. If the time is wrong, switch airplane mode off for a minute, or set the time zone by hand.

Silent Mode and Do Not Disturb are a different question

People often lump these together with airplane mode, but they work differently:

• The built-in Clock app rings through Silent Mode and Do Not Disturb on iPhone. On most Android phones, alarms are allowed through Do Not Disturb by default.

• Third-party alarm apps on iPhone are less predictable with the Silent switch. Depending on the app and the iOS version, Silent Mode can cut an alarm short or keep it quiet altogether.

If you rely on a third-party app, the safest habit is to turn Silent Mode off overnight. We wrote a separate guide to an iPhone alarm that didn't ring, and one for Android phones whose battery settings stop alarms, which is the more common problem there.

What happens when you cross time zones

An alarm in the Clock app is set in local time. An alarm for 7:00 means 7:00 wherever the phone thinks it is, so once the phone has picked up the new time zone, your alarm follows it: 7:00 in Lisbon, 7:00 in Tokyo.

That is usually what you want, with one catch: an alarm you set at home for "the first morning there" will fire by whatever time zone the phone believes it is in when the moment comes. If you set it before the flight and the phone updates on landing, you are fine. If the phone never updates (see number 3 above), it will ring by home time.

A checklist for the night before an early start

• Charge the phone, or plug it in next to the bed.

• Check the time on the lock screen is the local time where you are.

• Use a sound stored on the phone, not a stream.

• Turn Silent Mode off if you use a third-party alarm app on iPhone.

• Set a second alarm a few minutes later for anything you truly cannot miss: a flight, an exam, a shift. Two alarms are cheap insurance.

Airplane mode can stay on the whole time. It is not the thing to worry about.

How Waky handles it

Waky is built on the same principle as the Clock app: every alarm is scheduled on your phone, so it rings with no connection at all. An alarm set for 6:30 stays at 6:30 local time when you travel or the clocks change.

The missions mostly work offline too. Maths, typing and shake are checked on the phone, and push-ups and squats are counted on the phone as well. The one exception is the photo missions, which need a connection to check the photo. If there is no signal, that mission switches to maths for the morning instead of leaving you stuck with a ringing alarm. You still have to get up; you just do it with arithmetic. If you are travelling, a mission that doesn't need the camera at all is the simplest choice — here is how to pick one.

Your streak is saved on the phone first and catches up with your account the next time you have a signal, so a week offline does not cost you anything.

The bottom line

Airplane mode will not stop your alarm. A dead battery, a streamed alarm sound or a phone still on home time can. Check those three, and you can keep flight mode on and sleep.