why we get tired its true or only effect?

It actually all has to do with a chemical in the brain called seratonin. It is the chemical responsible for wakefulness, hunger, happiness and other things. During the day seratonin gets broken down into the chemical called melatonin, which causes the feeling of tiredness. That is a very simple way to explain it. Obviously it is much more complicated than I have made it sound. Also of course our bodies need to recover from stress and activity and that requires rest.