Mind - The biological clock

The strong proof that human brain has a biological clock within himself is a
proven fact. But how are we going to recognise it. Here are some proves from my understanding of "who am I?"
1. When we were going to school and college times. During exam days we will get up morning exactly in the time we want and we will do the task we thought of doing and go for exams. But we can't continuously get up at the same time on other days, why? Morning we get up and at the scheduled time the tasks start to be executed on exam day. But how come this one is getting possible in time of exams but not to strictly follow in long run?

2. When people from one geographical time zone go to other geographical time zone they find "jet lag". What is it? Why is it?

Post your comments about your understanding on this?
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