Many people know the feeling of opening their eyes a few minutes before the alarm rings, and the article says science has a clear explanation. The body has an internal timing system, the circadian clock, that can learn a person’s routine and begin preparing for wakefulness before the set alarm time.
This clock sits in the brain and regulates sleep and alertness over roughly 24 hours, but it also affects body temperature, blood pressure, hormone release, hunger, and mood. When sleep and wake times are consistent, the body recognizes the pattern and starts getting ready for the morning in advance.
A key part of that process is cortisol. Although often described as a stress hormone, cortisol naturally rises in the early morning and helps shift the body from sleep to alertness. The article says studies have found that people who know they must wake at a certain time can begin releasing cortisol about an hour beforehand, which is why many wake up spontaneously just before the alarm.
This happens especially on workdays or before flights and important meetings, when the brain is effectively on guard to avoid oversleeping, a phenomenon sleep researchers sometimes call intentional sleep. Usually, waking a few minutes early is a sign of a well-synced sleep schedule, but regularly waking an hour or two too early can point to stress, anxiety, depression, alcohol use, sleep apnea, or hormonal changes. The article adds that sudden changes to wake times, such as setting an alarm for 4:00 after usually waking at 6:30, are harder for the body to anticipate, while regular schedules, including on weekends, morning natural light, and less blue light at night can help train the body to wake more reliably without an alarm.
