f.lux is running when stuck in "daylight" mode after sunset. I can pull up the preferences and change them. That's how I exit f.lux.
I can't say if f.lux works sometimes, because I don't remember the last time I was working on my computer at sunset, but I can check.
Question: Can f.lux be affected by Windows' screen saver and/or power management settings? I can't get the display (Dell E173FPc, correct driver installed) to turn off when the computer is idle. The power button flashes orange (display off) then turns green again and the display shows the mouse cursor on a black background. (This is a problem I hope to take care of in the near future.)