Not having recently spent a lot of time looking at the sky at night, I don't really know how to answer this one, but let me try to cobble together something anyway!
Human night vision depends on the pupil being dilated as much as possible under low illumination levels. It also depends on the age of the observer and how well-stocked the retina is with a chemical called rhodopsin derived from vitamin A.
When you first go outside, your pupil is not well dilated having come from a well-illuminated environment, so it takes something like a half hour or so for it to enlarge to its maximum size; a bit longer if you are old rather than young. During this time, you ought to notice more stars begin to appear in the sky as your sensitivity to faint light levels slowly increases. You should also notice that the initially inky black sky should slowly appear to brighten as your eye becomes more dark adapted to the faint stars, and the presence of the Milky Way, and for a short time, the Zodiacal light which runs along the ecliptic plane from east to west.
If other amateur astronomers would like to comment on this brightening effect, please send me some mail about it!