Basenji is waking up screaming in he middle of the night?

[anp1107]

My basenji mix is 4 years old and I got him when he was 10 weeks old from a local shelter. He has always been a happy/healthy pup. He is mixed with a shiba inu and lab.

Earlier this summer my basenji mix was waking up in the middle of the night doing a very high pitch scream. He looks scared and disoriented and then wants to cuddle afterword. He does not twitch, jerk, or move his legs at all. He is awake and just laying in his bed. This scream is the most awful noise I have ever heard come out of a dog. The first time I heard it I thought someone was being murdered! He sleeps in the same room as me on the floor on a bed with blankets. It normally happens between 12:30 am-3:00 am. It has never happened when he's been taking a nap during the day. It happened about 6 times in June. Then suddenly stopped. I took him to the vet and they did blood work to see if his thyroid levels were off and if he had epilepsy. They ruled out both. My vet is baffled. Well now this past week he did it 3 times in one night and 2 other times later in the week. He has never had more than one "fit" in a night.

Does anyone know anything about this kind of behavior? I have no idea what to do. Could these be nightmares?