When a baby is about to fall asleep and heaves a great sigh why is that and what is it called?

The baby heaves a great sigh because he/she realizes it's doomed with you as a parent. * I have noticed this in breastfed babies in particular. I have no explanation other than complete satisfaction. A: Almost certainly a "clearing breath". A clearing breath is a deep breath that flushes CO2 from your system and increases O2 levels above what normal breaths do. This is a normal part of a healthy sleeping pattern in adults and children.