animals who hunt for food 12 hours are tired so take rest at night.

some animals hunt at night and work at night and then take rest at night.

Every living being has to work and live and eat food. 

Shrama or exercise will burn some calories. There should be a balance of what we eat calories vs what we burn. 

This is arogya lakshanam.

Lack of exercise causes obesity. If we overeat without exercise is not good. 

some people are think yet have good weight because of muscle weight.

some people look fat but they dont have energy.

Before in  olden days people used to work hard like pull water from well, cut branches to cook etc lift weight and sweat body and burn lot of calories. So everydays account got settled because of exercise, so there was no obesity problem.

If you work hard then you should definitely eat more and also should be hungry more.

We should eat as our activity level.

If we work more eat more, if we dont work or exercise then eat less to maintain ideal body weight.