Why you sleep through alarms when other people don't

Some people wake at the first beep. Others sleep through a phone screaming eighteen inches from their head, then get told they should "just go to bed earlier". If you're in the second group, there are three separate mechanisms at work, and they call for different responses.

1. Sleep inertia: the twenty minutes you don't get a vote in

Waking is not a switch. For a stretch after you open your eyes, the parts of the brain responsible for decisions and self-control are still coming online, while the parts that can reach out and tap a button are already working fine.

That gap is what the snooze button lives in. You are not choosing nine more minutes — the choosing machinery isn't running yet. It's why people genuinely cannot remember dismissing an alarm they definitely dismissed.

Sleep inertia is worse when you're short on sleep, and worse again when the alarm catches you in deep sleep, which brings us to the second mechanism.

2. Where the alarm lands in your sleep cycle

Sleep runs in cycles of roughly ninety minutes, moving between light sleep, deep sleep and REM. Being woken from light sleep feels survivable. Being woken from deep sleep feels like being dragged up from the bottom of a swimming pool — and is far more likely to end in a snooze you don't remember.

This is largely luck, and it's why the same alarm at the same time can feel completely different from one morning to the next. It also explains a frustrating pattern: sleeping longer can leave you groggier, if the extra time means the alarm now lands mid-cycle instead of at the end of one.

3. Habituation: your brain learned to ignore that sound

Brains are extremely good at filtering out signals that repeat without consequence. A sound you've heard every morning for eight months, which has never once been followed by anything genuinely important, gets classified as background — and background noise doesn't wake anyone.

This is why a favourite song stops working within a fortnight, and why people who switch alarm tones report a few good mornings before sliding back. You didn't get worse at waking up. The novelty ran out.

Why snoozing makes all three worse

A snooze doesn't return you to restorative sleep. It starts a fresh cycle your brain has no chance to finish, so the second alarm now arrives in the middle of one — the exact scenario from mechanism two. And each dismissal teaches your brain one more time that this sound can safely be ignored.

Research published in Scientific Reports in 2025, drawing on more than three million logged sleep sessions, found that around 56% of sleep sessions ended in a snooze, with heavy snoozers averaging roughly twenty minutes in that fragmented state each morning. That's not rest. It's an extra twenty minutes of the worst kind of sleep.

What actually changes the outcome

Given the three mechanisms, only some interventions make sense:

• A consistent wake time helps with cycle timing, and it's the single highest-leverage habit — though it takes weeks, not days, to pay off.
• Getting light and movement early shortens sleep inertia. This is the actual reason "get out of bed immediately" is good advice — not discipline, physiology.
• Putting the phone across the room works for some people and fails for others, because standing up, switching it off and getting back into bed is entirely possible while still asleep.
• Changing the sound regularly delays habituation, but only delays it.
• Making dismissal require thought is the one that targets sleep inertia directly. If switching the alarm off requires the part of your brain that's still offline, it cannot be done unconscious — and by the time you've done it, that part has come online.

When it isn't any of this

Consistently sleeping through alarms despite getting enough hours is worth taking seriously rather than treating as a personality trait. Sleep apnoea, delayed sleep phase syndrome, some medications, and simple long-term sleep debt all produce exactly this pattern, and none of them are fixed by a louder alarm.

If you sleep seven or eight hours and still can't be woken — or you wake exhausted no matter the duration — that's a conversation for a doctor, not an app.