Question

why do we feel lazy in the morning even after full night sleep?

Answer 1

This is mainly related to behavioural sciences -- everyday we need motivation to proceed with our day. It is part of human nature. 

Let's say you don't like going to school. Then, you will feel lazy, as you have no motive to get up. But, if you like school or some other activity, you will get up immediately as you want to do that activity.

As I said, human nature. :)