«قد يقام السبب مقام العلة عند تعذر الاطلاع على العلة»
A sabab may stand in for an illah when it is difficult to observe the actual effective event.
The author uses deep sleep: sleeping is not itself a physical discharge, but identifying whether the relevant event occurred during sleep may be difficult. In legally specified circumstances, sleep serves as the practical marker.
Substitution is therefore an administrable legal rule; it does not assert that the marker and the underlying event are identical in nature.