The body is active even during sleep at night, though not as the daytime. Heat production is not as high as during the daytime, so one uses bedding to cover the less heat production at night.
The reason why children move a lot is because their bodies are too small to store the heat, so they move their bodies a lot to supplement the heat even during night. This is why children tend to be untidy sleepers in general. Moving a lot during night is a good movement to produce heat in the body.
However, this does not produce enough heat, so if one takes off bedding, the body gets cold.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.