根据下列短文内容，在短文后的空格处填上一个恰当的词，使短文完整、通顺。

During the day we work and play. At night we sleep.  Our body rests and grows most while we  78 asleep.  79 the morning we are ready to work and play again. Children who are tired usually need more sleep. We can study better after we have  80  rest. Boys and girls who are eight or nine years old 　 81  ten hours of sleep every night.

Our body needs fresh air when we sleep.　 82  we do not get enough fresh air, we will feel tired when we wake up. We should    83  our windows at night, and we can have a lot of fresh air. Cool air is better  84  warm air. Boys and girls who want to be 　 85  must get pleny of sleep.