There must be something keeping track of time in our bodies and brains. I had a neat little lcd alarm clock that I bought as a 13 yr old with my own money, and I had it for almost 30 years. I became so finely tuned to that clock I used to wake up a few seconds before it went off and I would hold the button down. A microsecond blip of the alarm still snuck through but I did this at least once a week for years. Most of the time I would wake a few minutes before. It proved to me long ago that the brain can track time with uncanny precision, given enough conditioning.