Generally speaking, people perspire more than expected during sleep. This can be explained because heat qi, that is produced by the work of internal organs, is absorbed by various body activities, such as seeing and thinking, while being awake, but during sleep there are no such actions.
Especially, children perspire more than adults during sleep, because they are in the growth period. If they emit heat qi, taking off duvet or bedding, their bodies end up getting cold. This is called cold caught in sleep, and this is why kids get chilled easily while sleep.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.