Wakes up crying

Recently my son would start crying in his sleep. Most of the times his eyes would remain closed, sometimes he wakes up all the way.

He's such a mellow baby, and could go an entire day without fussing or crying, so when it happens it makes me sad that I'm not sure what to do to help. I usually just pick him up and rock him until he either wakes up all the way, or goes back to sleep.

Just curious to know if this happened to any of you ladies, and how you handled it.