When I go to bed, I can set an alarm and the tell myself to wake up before it goes off. I’ll wake up before the alarm goes at an average better than 9 times out of 10. It doesn’t work when I’m napping. It does t work if I’ve had a few drinks. But on a typical morning, I’ll wake up anywhere from 20 minutes to 2 minutes before my alarm.

However, if I wake up and check the time an hour or more before my alarm, what I can’t do is change the time I plan to wake up. It’s like the original time is locked in, and in a half-sleeping state, I lose the ability to change that time.

This morning I woke up an hour and 25 minutes before my alarm and decided, I’ll get up 30 minutes earlier. Closed my eyes and even dreamt about waking up earlier. Then my eyes popped open 2 minutes before the original time I had planned to get up, allowing me to shut off my alarm before my wife had to hear it. No extra 30 minutes for me this morning.

Despite my inability to change the set time during the night, I’m always puzzled and amazed at how I’m able set my internal clock before going to sleep. How does my body/mind know what time it is? If I woke up at exactly the same time every day, I think this would be more understandable. But I vary my wake up time by over an hour on weekdays and wake up another hour plus later on weekends. And I haven’t heard my alarm in over 2 weeks.

With respect to how we’ve evolved, the inventions clocks is such a new novelty, that there is no way it was a necessarily learned behaviour. Being sensitive to light outside our eyelids is a natural cue to let us know it’s time to get up. But the difference between waking up at 4:45 and 5:30, in total darkness, isn’t something our bodies should just know and be able to do.

How does our body know the difference? How are we able to control an internal clock? And how do we keep track of this while we are asleep, unconscious?