No, that is not it. It is the game itself that emulates day and night by making it bright or dark, and it happens several times a day. Don't you get that too when you are playing?
Yes, I get the day/night cycle. And caves are often dimmer then outside. But it never gets so dim that it becomes difficult to see anything. Even in the darkest parts of Eldevin there's never a problem. Just enough to add ambiance.
That's why I thought of the auto brightness feature on some graphics cards. If it decided to dim the brightness setting as Eldevin faded into night I could easily see it causing the problem described.
I might have the feature name wrong. Searching for 'auto brightness' brought up links for disabling this on Intel graphics. 'Dynamic Brightness' found this link regarding nvidia http://rog.asus.com/...items-on-screen. From that thread:
if you go into Nvidia Control Panel, go to "Adjust Video Color Settings", check "With Nvidia Settings:" and use full dynamic range, and disable "dynamic contrast enhancement." Restart computer and voila!! No more dimming on dark pages/programs!
Anyway, this was just a thought and may have no bearing on your issue. (BTW, the purpose of auto brightness is apparently to improve quality of blu-ray and HD.)