Our body grows most while we are asleep. The _____ children usually need more sleep. After we have plenty of rest, we can do our lessons better. Boys and girls about fifteen need nine hours of sleep. If we don't get enough _____ air, we will feel tired when we wake up. While in bed, we mustn't _____ our head. Our lungs need to get enough air, so we'd better open our window _______ and let fresh air in. Scientists say that cool air is better than _____ and sleeping in heated rooms does no good to our health.