You may think that lying in bed until 12 o'clock on avi the weekend is good for you, but you are wrong. Your biological clock tells you when you are at tired. It also tells you when to wake up, and if you upset this by sleeping late on the weekend, it gets confused and you feel even more tired. A particular chemical is responsible for waking you up. It starts to work from 3 or 4 a.m. until 11 a.m. If you sleep any later than this, you prevent the chemical from working, which in turn prevents you from feeling wide awake. You may wonder what being thirsty has to do with being tired, but when you exercise or play a sport, it is very important to drink fluids. You feel thirsty because you have lost liquid from your body, and even with a loss of 3%, your physical ability is affected. You should drink at least eight glasses of water a day.