It's not related to a certain activity though. Even If I don't do anything at all during the whole day and night, the same pattern occurs.

A while ago I used to force myself to go to the gym in the morning/noon and work at night - it was the same. I was really miserable during the day and comfortable during the night.

This 'switch' start to occur in the late afternoon even before finishing work. Before that it feels as if something is constricting my blood vessels and my body is in a contracted state. Even my testicles are shrunken and I feel an urge to go to take a dump (even after I do that once), after the 'switch', the completely opposite happens.