Why, when you lie on the couch all day, do you start to feel tired in the evening? And why does it disappear from a little physical activity?

So everything is too commonplace: during training (even if you just sit down at least 100 times per approach), the heartbeat increases, this leads to an acceleration of blood circulation to the brain. The production of adrenaline into the blood also contributes to "wakefulness".
If you lie on the couch all day - the blood thickens, there is little oxygen, the brain is not able to work, as after an active workout. Even if you solve a crossword puzzle or read a book on the couch, your brain will feel a lack of "fresh blood" and air.

A person is predisposed to movement, because a "sedentary" lifestyle (modern) is unnatural for us.

Perhaps you mistake for fatigue muscle stiffness that develops with prolonged immobility.

This stiffness is uncomfortable, but enough to "disperse" and the discomfort will disappear.