At night there is not only a reduction in visibility, but also in the light quality. Surrounding cultural lighting can be misleading at night as well, or cause glare. Depth perception suffers and as a result more cross-referencing is required.
Flying in Canada, often in very sparsely settled areas, the effects are even more pronounced and often we wind up flying IFR on nights with no cloud.
The safest approach I have found is to treat night VFR like it is ALWAYS marginal VFR and be very careful.
About the altitude, part of the problem is also geting accustomed to a particular height. Students I have had that have never flown seized-wing a/c are less accustomed to the height and like it less. From my military time, we all started on jets and so were never really bothered by it. Its actually a lot of fun to see the world from on high once in a while.
Fly safe, and have fun.