"I'm not tired!" protests the child, at the prospect of impending bedtime. To which you may reply, "Au contraire! Your oscitancy betrays you!"
Or not. We understand that such word nerdery may not go over well in all households. Regardless, oscitancy is a useful word. Its adjectival relation, oscitant, is also available when you want to describe one who is either yawning with drowsiness, or, less charitably and more obscurely, one who is lazy or stupid. The words are Latin in origin, from oscitare, "to yawn." That word's roots are os, meaning "mouth" (also at work in osculate, "to kiss," and in its variant or form, oral), and citare, "to put in motion," which is antecedent to such terms as recite, resuscitate, and excite.