«إذا نام مضطجعًا استرخت مفاصله»
When a person sleeps reclining, the joints of the body relax.
The book uses this formulation in discussing sleep and the nullification of ablution. Reduced bodily control during reclining sleep is offered as an operative characteristic linked to the ruling.
The present objective is to see how a characteristic mentioned in transmitted material can inform identification of an ‘illah, not to settle every practical question about sleep and wudu. The status and legal interpretation of specific narrations can warrant independent analysis.