Sweat has a role to keep body heat in a certain condition. So, it is quite reasonable to have perspiration when being active in daytime. Even during sleep, the body is active quietly, people sweat lightly.
However, abnormal sweat during sleep is the matter. This indicates that even during sleep, abnormal heat is produced somewhere in the body, in other words, there exist weak points in the core of the body (internal organs) that can control heat during night. The idea of internal organs here is a broader concept than those in Western medicine; for example, it is said that bone diseases belong to Kidney, and this idea is broader than kidney in Western medicine.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.