Why do we sometimes wake up a few minutes before our alarm rings?

The human body has a built-in timekeeping system called the circadian rhythm. It is governed by a group of cells in the brain, centred on the suprachiasmatic nucleus, which uses signals such as daylight to keep the body’s roughly 24-hour cycle aligned with the outside world.

When you wake up at the same time every day, your brain learns. In the hours before your usual waking time, the body slowly prepares for wakefulness: e.g. levels of the stress hormone begin to rise and body temperature increases. Over time, these changes can help you wake naturally when you expect to.