why do we require energy while sleeping

Even while we are sleeping, several of our body functions still go on in order to keep us alive. For eg - The heart keeps beating and the cardiovascular system continues to function. Endocrine glands still function. Growth Hormone is secreted more while the person is asleep. The nervous system and urinogenital system continues to work.  Our respiratory activity goes on. The body maintains its basal metabolic rate.

All these activities require energy, hence we need energy while sleeping.