In book 2 of WWR, Schopenhauer says “If now every action of my body is the manifestation of an act of will in which my will itself in general, and as a whole, thus my character, expresses itself under given motives, a manifestation of the will must
be the inevitable condition and presupposition of every action” (WWR, vol. 1, bk. 2, §20). What he means by this is that will is the driving force behind all human actions and experiences; every action is an expression of will. He believes that our will entails desires and motivations, and thus the manifestation of will is a condition for every action. Without a will, action would be impossible, so it makes sense that “teeth, throat, and bowels are objectified hunger” (WWR, vol. 1, bk. 2, §20), is used as an example. If this is true, then the same must apply to mouths and eyes (yawning) being objectified tiredness. We desire and have a will for sleep, so the manifestation of this will present itself as a yawn and thus leads to the exhibited action.
The manifestation of will is a presupposition for every action; before we can perform any action, our will must be present (WWR, vol. 1, bk. 2, §20). This is why hunger can be objectified through teeth, throat, and bowels, and tiredness through mouths and eyes (yawning).