The Lan Sleeping Posture is just because they don't believe in mattresses, sorry to WWX's sex life.
@Betty WWX internally debating “every day means every day” against the joint pain and coming out deeply conflicted
@Betty yeah i don’t think he was getting significant amounts of sleep there, or at least not any outside of the “so tired i pass out the second i sit down” variety which is not really conducive to noticing the presence/absence of mattresses
@Satsuma I mean, from what we know of the Demon Subdue Palace, WWX is either used to it, or has figured something out.
(I mean, actually, he may have dealt with a lack of mattress there by simply not sleeping, it does seem uh... congruent with the facts we know about his time there.)