Based on [Robert Kooimas](http://www.evl.uic.edu/rlk/) [GPU Day-Night Illumination Tutorial](http://www.evl.uic.edu/rlk/cs594/final/) I created a simple day-night cycle skydome setup to put in your projects. It is easy to use and is really fast. I could easely replicate the shader in Node editor, but it requires Suns XYZ position.
I also noticed that it fade to night for a few moments when the sun is on the highest point (high noon) and when it’s night and the sun is on the lowest point it fade to day for few moments, but at the moment I can’t find the exact reason for this.
It’s for UPBGE only for now, because I started with it, and if you try opening it in regular Blender the materials are all messed up, but I’ll try to put together a BGE version too.
The problem you’ve been having with the sky turning to night in the middle of the day only happens when the sun lamp is exactly on the top center of the half sphere, making the sun move so it never aligns with it bypasses the problem.