I think I may have found a cause/solution. I had the same issue followed by regular ctd which I attributed to save bloating and corruption so installed the save hotkey mod to avoid overwriting saves files. This issue started again on my current playthrough, so I went back to the last save when I could sleep without CTD and started testing to see where it became corrupted. It seems that as soon as the Dawnguard quest starts so do the ctd, I tested this successfully a few times to be sure that it was the quest. I then tried to start the quest by console command on the last save I was able to sleep but the quest didn't start at stage 5 and i was still able to sleep. So I went to the nearest town in order to receive the quest from a guard (as I was previously doing during testing) and ctd again. I repeated but then I checked to see what stage the quest was and even though the quest had started and should be at stage 5 it remained at 0. So I set the quest stage to 5 and I was suddenly able to sleep without ctd. I'm not sure whether the cause is a conflict with a mod or whether my game will remain stable as I haven't continued to play, but I'm fairly certain that I've found the cause.