Suggestion - Time Settings and/or Delay feature
While the sunrise/sunset is a good start, and the ability to ctrl+drag means that you can alter the sliders in a shift of your regular daytime to suit irregular schedules, I (and I imagine others) are in a bit of a 'special' spot when it comes to our sleep schedules. I'm in the unique situation where I have to split my sleep (several hours in the early afternoon and several more in the evening) in order to work and to function during the normal hours that people are active and free in my area.
I would love to have an 'advanced' setting option or the like with drop-downs where you could set the time you'd like it to begin or finish fading/brightening. Especially if you could set more than one so that those of us who can't get our sleep all at once could still use the program.
Heck- I'd be delighted if there was a 'Delay' you could set for X hours via dropdown or the like, so that I could set it to dull during my afternoon sleep and then just flick it off for six hours or whatever in the evenings. (The setting sun tends to be lack enough for me then).
Thank you so much for reading, staffers and consumers! And anyone who wants to support the idea is more than welcome to reply with a comment along those lines just to make it easier to get a look at how others feel about the idea.
Manual control of the schedule is an often-requested feature. I think the developers are working on a new version that addresses some of that, no idea when it's coming out.
Just a couple of comments about your situation. I'm assuming you have a regular schedule, not doing shiftwork where you change "time zones" every few weeks? In other words, a bit like in southern countries where they have a siesta in the afternoon.
I'm not an expert, but as far as I understand it:
Your body starts producing melatonin after dark, it's one of the things that make you feel sleepy. Exposure to blue-green light at night can supress melatonin production and make it more difficult to fall asleep. It can also cause your circadian sleep pattern to shift later, similar to jet-lag from travelling eastward. The body's response is slow, occurring over an hour or more of light exposure. F.lux is designed to prevent this.
During the day though, the body doesn't produce melatonin, even if you normally have a siesta in the afternoon. So melatonin supression isn't relevant. And early in the day, blue-green light has the opposite effect on the circadian rhythm, shifting it so you sleep earlier at night. There's a crossover point, somewhere in the late afternoon or early evening, where it has no effect. In other words, the things that f.lux is mainly meant to deal with are not an issue in the afternoon.
There is a third factor, which is a general "alerting effect" of bright light in the daytime. The response time is faster - a short exposure can stimulate the brain and make you more alert, but it's only temporary. There hasn't been as much research done in this area.
In short, it won't hurt to use f.lux in the hours before your afternoon sleep, but it's much less critical than it is at night. Just turning off the computer and closing the curtains shortly before you want to sleep may work equally as well.