Disciplining your 15 month old

Is it not the cutest thing in the world when you hear your fifteen month old baby girl awake in her crib, when she should have been sleeping an hour ago, singing the 'Head, shoulders, knees, and toes' song? Obviously in her own version, but seriously!!! How do you put on that serious 'mom' face and tell her she needs to sleep or else...when she's patting her head and making all the motions???