This is definitely something that needs to be addressed - priapism lasting longer than 4 hrs can permanently damage the erectile tissues of the penis, to the point you could be permanently unable to achieve erections again, even w/ Viagra, Cialis, Levitra, etc. There are certain medical conditions that can cause this - sickle cell anemia, for example. If I were you, I would not let this go - keep hounding Dr's for an answer. Have you seen a Urologist? Keep bugging the Dr's till you get an answer, and please check back in here & let us know what was found...
Oh ya, definitely talk to your doctor! Priapism is the problem. We'd like to think you were just having a lot of sexy dreams but if the erection isn't coming and going during the night, no, that's not the case. Priapism can be ischemic or non ischemic. With ischemic, you are usually soft at the tip but rigid in the shaft and with non ischemic, you are usually erect but not fully rigid. Any of this ring a bell for you? There's different reasons for each one. Let us know which sounds more like you. https://www.mayoclinic.org/diseases-conditions/priapism/symptoms-causes/syc-20352005