If you are meaning more sort of “why” we get tired then generally its when we either get too little sleep or undertake strenuous physical or mental activity. However you tiredness can also be a symptom of various medical conditions – like anaemia…and being pregnant.
You would think that for something so common we would understand it better. People have been studying sleep for a long time, but it is really a very complex process, and sorting out cause from effect is hard. We know lots of things that change in the body when we go to sleep, but what causes what is hard.
Certain brain chemicals like melatonin and seratonin increase, hormones are released that change how actively your muscles absorb ernergy, the nervous system goes into a less active state … and you feel tired. If we knew exactly why we could do a much better job at dealing with things like driver fatigue and jetlag.
Our bodies do need to rest in order to operate efficiently. In particular our brains require time to rest and process information when we sleep. We also do a lot of healing (like when we are sick) when we are sleeping. So being tired is our bodies way of telling us we need to take some down time to regenerate! Our body can manage when we feel tired by releasing hormones as the others have mentioned. It does so in response to symptoms of fatigue!