I've been sober a little over two weeks and for the first week.... sleep almost entirely eluded me but in the few moments that I could sleep.... HORRIBLE nightmares.
After doing some reading.... (and I'm not an expert so feel free to take this with a grain of salt)..... It seems that a few things go on with heavy drinkers.
1) For me, I never really "went to sleep".... I typically would kinda/sorta pass out. That isn't to say I would black out, but the alcohol absolutely contributed to me going to sleep in a major way.
2) Based on what I've read, when I stopped drinking my brain was still looking for alcohol as a catalyst to help me slip into sleep. Without it, my brain would stay awake waiting for it, and when the booze didn't come, my brain wouldn't "shut off" for sleep.
3) Further, when I could finally fall asleep (for about 30 minutes at a time) my brain, still desperately waiting for the chemicals in booze, was in a sort of "desperation mode" which translated itself into horrible, desperate dreams (including but not limited to a bizarre one where there was a shark on the floor in my living room that I was trying to kick away from eating me).
I still have trouble falling asleep but now when I do finally sleep, it is relatively sound and for the most part, the nightmares have ended. I would suggest that when you have a few minutes here and there, do some reading on the medical/mental health side of quitting drinking as it might help prepare you on what to expect. For me, understanding the reasons why I was/am going through sort of helps me cope (although it still aint easy).
Good luck.