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When someone regularly wakes at the same time, the body's circadian system can begin preparing for awakening before the alarm sounds. Hormonal and physiological changes associated with waking can start beforehand. This may help explain why people occasionally open their eyes just minutes before a familiar alarm. The ability isn't perfectly accurate and becomes less reliable with irregular schedules or insufficient sleep. Still, the body keeps its own biological schedule even without looking at a clock. Sometimes, the internal alarm gets there first.
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