Touch a creature to cast this spell. The creature rolls a constitution check against your spell DC. If they succeed, they are unaffected and immune to this spell being cast on them again for 10mins.

If they fail, they involuntarily yawn until the end of your next turn. During this time, they are semi-conscious, aware of their surroundings but effectively Stunned - unable to take actions or reactions, cannot move, and speak only falteringly through their yawn. They automatically fail strength and dexterity saving throws, and attack rolls against them have advantage.

If the creature takes damage, or another creature uses their action to slap or shake the target creature, or the spell ends, the target creature becomes unstunned and is immune to this spell being cast on them again for 10mins.

While under the Yawn spell, the target creature is extra vulnerable to the Sleep spell. If a Sleep spell is cast such that the target is in the affected area, they immediately succumb to it, before any other creature, regardless of the regular requirements of the Sleep spell. 

If the Yawn cantrip is cast outside of combat, on a creature that is neutral/friendly and relaxed/unsuspicious, the Yawn takes immediate automatic affect, and the Constitution check will instead determine:

If they succeed: the creature remains awake, unaware that a spell is the cause of their yawn, and immune to a repeat casting for 10mins.

If they fail: the creature falls in to a natural sleep by the the conclusion of the spell - in to a deep sleep if it's during or near the creature's regular sleep time, or a light doze if otherwise. The creature can choose to fail the Con check if they wish.