Nightmare

Case had a baby nightmare last night and it was so sad to see. He was upstairs sleeping and all of the sudden he started screaming like he was hurt. I flew up the stairs and he was in his crib eyes shut screaming his little head off. He wouldn’t stop even when I picked him up. I tried to wake him up and that helped a bit, but he was still clearly upset. So I went to the rocking chair in an effort to settle his little nerves, but even while nursing Case would drift off and start to pucker up his little lips and cry out. It broke my heart! Finally after about an hour of comforting, he finally settled into a peaceful sleep and woke up this morning with a smile on his face. Thank goodness! So my question is, what do little babies have nightmares about?