Rest and relaxation are just as important to our body as exercise and healthy eating. If you don’t get enough of them you can make yourself ill. Your body is a bit like a battery. If it is not allowed to recharge itself through sleep and relaxation it may stop working properly. Lack of sleep affects our ability to concentrate. When you dream, your brain may be clearing itself out and preparing itself for the next day's thinking. During your teens, you need at least eight hours' sleep a night. If you stay out late, try to go to bed early the next night. The effects of lack of sleep can build up over several days. When you are awake you make many demands on your mind and body. Your body needs a period of rest to repair itself and prevent it from getting worn out. During sleep, substances called hormones stimulate body tissues to grow and repair themselves. If you have troubles getting to sleep, try reading a book to calm your mind or take some exercise and avoid tea, coffee or food before bedtime. A hot, milky drink may help you relax.