What else happens around the same time?
Is it a mild malar rash? Or if you could be photosensitive, indoor lighting (t.v., monitors and phone screens, incl.) the light can cause your skin to burn (uva's and mostly uvb's damage dna & increase inflammation.)
Symptoms can sometimes pool toward the end of the day and cause inflammation/malaise to spike. I've always wondered about this one myself b/c I've vascular flushes/flares and coughing fits that you could set a watch by--a few hrs every evening. The worse they are, the more I gasp for breath, so I'm never w/o a piece of soft candy that I can suck on until the fit hopefully passes.
I look forward to hearing how your mystery's solved