Sorry for the belated answer - had some hectic time at work :( I will try to be analytic about it and hope you don't mind the lengthy comment. :)
In general, I think you are very close to getting very realistic with your pieces. What needs a bit more balancing is the contrast between shadows and highlights.
We could divide all lightsources into two types: ambient and directional. While the ambient light would have a form of a omnipresent diffused light, under which every element is lit rather equally, the directional lighting would be much more concentrated and (uh) directed onto a particular spot.
What I think is the issue here, is that in your two pieces here, the background seems to be under an ambient light, and the character under directional light.
The practical outcome is that both of the characters have their highlights condensed and thus the contrast between shadow and highlight is too high.In the daylight scene, I would be tempted to make the shadows lighter - the dark shadows right now indicate a much darker background than already painter. In the night time image (and yeah, the image came in one of my workshops! :)) I would dim the highlights a bit, as well as deepen the shadows (however, a directional lightsource placed above the character's head would look very cool here).
Aside from that, you might want to vary the highlights a bit. For example, I think that some pure bluish tones in the night image would look interesting.
As for the eyes image you postd a link to - no crits about it, the image is really good the skintones rich and the whole thing looks realistic, great job :))
By the way, the night time image is really good and has
really a lot of potential - I would greatly encourage you to finish it!
The facial anatomy is really original (i love her fatures btw) and with
just a bit more pushing, this could be awesome. :)
Well, I hope I managed to help you. If you'd like to know anything else, please let me know, I promise I'll take faster to reply this time! 