Why do I wake up right before my alarm?

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Sometimes I wake up a minute or two before my alarm is set to go off. Is this normal? And why does my body do this?

There’s little research into how common this experience is and why it happens. But it seems to be “a genuine phenomenon” that many people report, according to Russell Foster, head of the Sleep and Circadian Neuroscience Institute at the University of Oxford in Britain.

In a telephone survey published in 1997, for instance, researchers from Iowa and Minnesota randomly interviewed 269 adults mainly in the Midwest. About three-fourths of those interviewed said they sometimes woke up before their alarms, and just under one-fourth said they woke up so reliably that they never had to use an alarm.

After the research team published a newspaper ad asking for people who always or regularly woke up at specific times without using an alarm, they invited 15 of those respondents into a lab and tracked their sleep for three nights. They found that five of the 15 awoke within 10 minutes of their target wake-up times all three times.

Nobody knows exactly how or why the body is able to do this, but researchers say that our biological clocks, which keep track of time, have something to do with it.

Just above the optic nerve in the brain is a master clock called the suprachiasmatic nucleus, said Dr. Ravi Allada, a neurobiologist who specializes in sleep and circadian rhythms at Northwestern University.

This clock synchronizes and coordinates our body’s circadian rhythms, which help us prepare for things that happen at various times of day — such as falling asleep at night and waking up in the morning.

One way our body does this is by sensing the levels of light around us, Foster said. Special cells in our eyes detect changing light levels, such as right before and at dawn — even through our eyelids when our eyes are closed, he said. These cells probably don’t tell our bodies precisely what time it is, but they may communicate that we’re approaching the time we normally get up.

This triggers changes — such as increases in the hormones cortisol and adrenocorticotropin, as well as in blood pressure, Foster said — that help us prepare for activity.

What about the times you’ve woken up right before your alarm when you’ve had to be up much earlier than your body is used to, such as to catch a flight or attend an important appointment?

Instead of waking up based on what time it is, Allada said, our bodies may be waking up based on how much time has passed since we went to bed — working almost like an hourglass. If we go to bed knowing we must be up in 4 hours, something may help to ensure that we wake up after 4 hours.

If our bodies are so good at sensing the time, why don’t we always wake up just before our alarm? And why is it that some people never wake up before their alarm?

Foster isn’t sure. It’s possible that when you’re especially tired, your body’s need for sleep overrides its biological clock, he said.

Or sometimes, if you feel nervous about waking up on time, stress may cause you to wake up earlier than you’d like, Allada said.

There are still far more questions than answers about why and how our bodies sometimes wake us up before our alarms.

But to maximize the chance that you’ll rouse on time on your own, Foster said, it can be helpful to set your alarm for the same time each day so that your body gets used to waking up at a regular time.