Is it possible to slowly, smoothly fade/change lights over a period of hours?
My use case is to implement circadian lighting.
I would like to say, e.g., at 6 am the light is at 20% brightness and 2700 Kelvin, at 12pm it is 100% brightness and 6500 Kelvin - and in between, it fades smoothly over the whole time so that the change is so slow that it cannot be noticed.
Is this currently possible?
Yes, but not with WLED alone.
You will need a home automation software like HomeAssistant or similar.
I’d be interested in a standalone solution. Everyone can do it with additional home automation software, but that is not robust enough for my purpose (e.g., when the WLAN is not there all the time).
if I understand it right, what we’d need would be a checkbox:
Smoothly transition between time-controlled presets
It looks like the existing feature
Nightlight function (gradually dims down)
goes in a similar direction, maybe some code could be re-used.
Of course but you will need to write a usermod for that.
Use Sacn or Artnet protocol, And use. third-party lighting controller - Lot out there, just google DMX software
He said stand-alone solution. 
If you wanted to try some effects with current code a playlist of the 12 Temps you list, setup as presets might work.
Ver 0.13.0-b2+ code has playlists builtin, with transition and play times in 1/10s up to a max of (my reading of the code) 65535 (uint_16).
That should give you 6553 secs or 1.82+ hours of transition in and again out of each playlist preset.
You can easily make 12 different presets for each colour temp and trigger the whole thing based on Sunrise and/or Sunset.
Might take some fiddling to get what you want, but it looks feasible.
Just my $.02
Good idea but would only work for a short period of time (due to Earth’s inclination and change of daytime length) or only in equatorial places.
Don’t see why a linkage to the Sunrise/Sunset functions built into WLED won’t correct reasonably well.
Especially given that those functions are corrected by the entering the user’s Lat/Lon.
I would expect some additional offset to be required for extreme Summer and Winter hours of daylight differences (if you need to correct that last 20mins), but Civi/Astronomical tables of Sunrise/Sunset times work quite well based on the same principles…
It is not the offset but rather playlist entry duration that will be causing problems.
I.e. at latitude I live the length of day differs by 10+ hours (summer/winter solstice).
I see where you’re going if the transition(s) need to vary with Total hours of Daylight.
In that case you’ll need a usermod or at least something else to trigger the playlist changes based on Today’s hours of daylight.
Might possibly be the actual requirement, although the OP specified something much simpler.
Indeed, Playlists seem to be what I was looking for and should work at least when using fixed points in time (not linked to the actual sunrise and sunset). Thanks a lot!
I can imagine different usage scenarios:
If there is enough general interest in this topic (including using concepts like civil dusk and dawn - civil twilight is the period when enough natural light remains that artificial light is not needed), maybe someone will consider writing a usermod at some point?
What you can do is combine Playlists, Presets and Time controlled presets.
First, create 2 playlists to simulate sunrise and sunset (with a few Presets gradually increasing/decreasing brightness and color temperature if necessary), then have a Preset with daylight set-up and use it as an end preset in sunrise playlist, and finally an Off Preset and use it as an end preset in sunset playlist.
Next, add sunrise Preset to Sunrise controlled preset (adjusting start minutes as necessary), and sunset Preset to Sunset controlled preset.
Unfortunately adjustments only work +/- 59 minutes in current implementation
