摩訶止觀 4.179

4.179

調眠者，眠是眼食，不可苦節，增於心數，損失功夫；復不可恣。上訶蓋中一向除棄，為正入定障故。此中在散心時從容四大，故各有其意。略而言之，不節、不恣，是眠調相。

As for regulating sleep: sleep is the nourishment of the eyes. One must not restrict it painfully, which would multiply mental factors and squander one's efforts; nor may one indulge it. Above, in the section on rebuking hindrances, [sleep] was discarded outright because it is an impediment to properly entering absorption. Here, while in a dispersed state of mind, one accommodates the four great elements; thus each [context] has its own rationale. In brief, neither restricting nor indulging is the mark of regulated sleep.