The topic of the possibility of light from the viewfinder falling on your dslr sensor seems to be a topic that is either unclear/controversial or overlooked altogether. However, it is something everyone should be aware of!
Light leaking through the viewfinder; Nikon D5100
When testing the thermal noise in dark frames on the Nikon D5100 vs Nikon D7000 vs Nikon D600 (more on that later ;)) I ran into the issue of having a clear light band on top of the image. This was happening with the D5100 and I’ve seen it in the past with a Canon 1100D, so I recognized the problem: Light coming in from the viewfinder is falling on the sensor! After covering up the viewfinder the problem went away, so for sure this was the cause.
Looking around on the internet for more information of the viewfinder light leaking problem with the D5100 in particular, I ran into some different experiences; some say they don’t see the problem at all, often it is mentioned that this problem occurs (most often) with the entry-level dslrs.
Looking at the experience of other D5100 users it seems this is a problem that is not consistent per model, but most likely something that can differ from copy to copy.
Cover the viewfinder on your dslr
Obviously the solution is just covering the viewfinder when you are shooting with your dslr at night. You’re not using it anyway, so you should just do it. If you want to be sure your model has this problem it’s easy enough to test it out yourself; just take a view 5min. exposures with the lens cap screwed on the body and shine a light (or put it in the sun) on the viewfinder. Examine the frames (stretch if needed) to see if you see a light band across the image.
I would love to hear your results in the comments below. Is light falling on the sensor from the viewfinder with your dslr body too?