system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 I never wake up at 9am but if I set my alarm for 9am I will wake up a couple of minutes before. how does this happen?
gpt
You body has its own internal timekeeping (most creatures do). In particular there’s your daily ‘circadian rhythm’ which involves two 24-hour clocks in your brain (suprachiasmatic nuclei in your hypothalamus). These keep a steady tempo. Just like a watch, they need to be told what your timezone is so that you can wake during the day and sleep during the night (instead of the other way around). So your body responds to information like light, which affects hormones, to tell your timers when daytime is. This means your brain has a way of ”knowing” what time it is. It therefore can subconsciously coordinate with your alarm clock. The process is not sudden, it happens gradually. There accuracy is not 100%, so your body can be a bit early or a bit late. But if you tend to feel anxious or stressed (and if you hate alarms) or if you’re a light sleeper and there is environmental information (like birds, sunrise, traffic, etc), the tipping point can happen just before your alarm. It is possible for you to consciously intend to wake at a certain time without the alarm even existing, and your brain will comply. Likewise, if you know it is the weekend, your conscious intent can make your brain let you sleep in. It’s not a hard and fast rule for everyone, but the effect is certainly there.