Why does she cry in her sleep?

Carrie • 💕46 1/2 yr old Mom of 6 Sunshines🌞, 1 Angel👼🏼 & 1 Rainbow 🌈

All day she's happy. She really is! 9 months 9 old now and rarely is she without a smile, except in the car seat most consistently lol. But at night when she sleeps, she cries briefly, just a few seconds every now and then she let's it out. I'm a thinker and it makes me wonder why or what it is that makes her cry like that in her sleep like this... Anyone else ponder things like this? & does anyone have theories or answers lol.