I found nightmare to be more reliable than shiva's sleepga.
I found this as well. I find nightmare to be able to sleep things that ES-Sleep as well as Repose cannot. Absolutely love this pact and use it quite often when in a bind.
I honestly don't believe Our version of nightmare does Any DoT at all. While i can't say I've tried this on any really low level mobs to prove it, I know that it doesn't do any noticeable damage to EP or higher (checked to a 75). I'll try to verify this for sure on a bunny next time i'm on.
Also to answer the question about mobs regening while slept with nightmare I can say YES they will certainly regen. This was verified on Kinepikwa (Taster's Cape NM). He casts poison on himself to gain a VERY high regen, fairly certain that its probably the highest form of regen in the game as it will take him from 50% to 100% health in a matter of seconds and will do so while slept. It was mostly a matter of luck that I got my nightmare in on him at all as we were battling with Aqua Fortis and Diabolos was whacking at him. Luckily i got the timing right and released so that sleep stuck.