How many night hours is enough?
If IFR operations in real IMC is challenging enough by day, try dealing with it after dark! IMHO, not enough emphasis is given to this most demanding aspect of flying. As for me, I happened to have a NVFR Rating with quite a few hours before doing my Instrument Rating, but the school I trained with provided for only one night IFR exercise in their syllabus (a grand total of 3-&-a-bit hours). Shortly thereafter I was signed off as licensed to thrill. Talk about being thrown in the deep end! A few years later when I had gained some experience and was teaching multi/IFR myself, I discovered nothing had changed.
I don't think there is anything much more challenging (and potentially dangerous) than conducting a circling approach in marginal weather - at night. I for one am glad that I had some experience from my NVFR Rating to help ease the transition somewhat, but all training providers should be giving more serious consideration as to how they train students to deal better with this most important area of IFR operations. One night IFR nav just doesn't cut it.
My advice? Take every opportunity you can to gain experience at night - VFR or IFR. Needless to say, you should see a dramatic improvement in your performance by day, although that's not the reason why you do it.