f.lux Screen Filter: What It Does to Your Screen, and Whether It Works

What the f.lux screen filter actually does

f.lux describes itself as software that makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. Its setup instruction is equally short: tell f.lux what kind of lighting you have and where you live, then forget about it. From there it calibrates the display's color temperature from sunrise and sunset at your chosen location, shifting gradually to a warmer color after dark and restoring the original color in the morning.

The mechanism matters more than it sounds. f.lux rewrites the color values on their way out to the display, so every pixel arrives a little redder and a little less blue. The screen itself is a passive party in this. Nothing inside the panel changes: not the backlight, not the on-screen menu, not the picture mode. CircadianShield works at the same layer, which is why the honest parts of this page apply to both products rather than only to the competitor.

Here are the nighttime presets f.lux ships, taken from its own FAQ, so you can see the range before you install anything:

Beyond the color schedule, f.lux includes a film watching mode, a darkroom mode, Philips Hue LED lighting control, and an inverted-times option for night-shift workers. The same team publishes the f.luxometer, a separate reference tool for measuring the spectrum and melanopic response of a light source, which is a genuinely useful thing to have around whichever app you end up running.

Is f.lux software free?

Yes, and this is the clearest thing about it. f.lux lists the software as free for macOS, free on Windows for personal use, and free for Linux, downloaded from justgetflux.com. Corporate Edition and site licenses are sold separately, so if you are rolling it out across a company, the free download does not cover you and you should read the licensing before you deploy.

Some context on where it comes from, because a free download from an unfamiliar site makes people nervous. f.lux was released in February 2009 by Michael Herf and Lorna Herf, and it is developed by F.lux Software LLC. It has been around and maintained for more than fifteen years. Download portals also host it, but the developer's own site is the one to use.

CircadianShield is on the other side of that line and there is no point pretending otherwise. It is $47 one-time for up to 2 computers, with free updates and a 7-day free trial on Mac and Windows, card required. The comparison is not free against paid in the abstract. It is whether the specific gaps described further down this page are worth $47 to you, and for plenty of people they are not.

Does f.lux actually work?

Split the question in two, because it has two different answers.

Does the screen filter do what it claims? Yes, and you can verify it yourself the first evening. Warming a display's color temperature genuinely lowers the short-wavelength light it emits, because the blue subpixels are driven less hard and the red ones relatively harder. That part is measurable, and the f.lux team publishes the f.luxometer for measuring exactly this kind of thing.

Does it improve your sleep? Here the evidence is thinner than the marketing around blue light suggests, and CircadianShield would rather give you the real state of it. Wikipedia's summary of the research states plainly that f.lux itself has not been scientifically tested to determine its efficacy. It cites three findings that are worth reading together. A 1996 study linked reducing exposure to bright blue light at night with increased melatonin secretion. A 2018 study found that changing the spectral composition of self-luminous displays without changing their brightness settings may be insufficient for preventing impacts on melatonin suppression. And a 2021 study of Apple's Night Shift in 167 college undergraduates found no effect on sleep latency, duration, efficiency, or wake after sleep onset.

The thread running through those is that color on its own, at whatever brightness you happen to be using, is a weak lever. Both intensity and spectrum feed the melanopsin response in the eye. That is the reason CircadianShield calibrates against Melanopic EDI under CIE S 026:2018 rather than against a Kelvin number, and why it pairs the color shift with dimming instead of treating warmth as the whole job. CircadianShield is a wellness app, not a medical device, and it does not claim a proven sleep outcome either. The full reasoning is in the CircadianShield science page and in the melanopic EDI explainer.

Is f.lux actually good for your eyes?

A warmer screen is a reasonable thing to try for evening comfort, and plenty of people find 2700K far easier to look at in a dark room than 6500K. If that is your experience, f.lux is doing something useful for you and you do not need to overthink it.

What the f.lux screen filter cannot help with are the two causes of screen discomfort that have nothing to do with color, and this is where a lot of people get stuck trying warmer and warmer presets with no improvement.

Brightness. f.lux does not change it. Its own FAQ states that it changes the color temperature of your display and does not adjust brightness. A warmed screen frequently feels dimmer, because your eye is less sensitive at the red end of the spectrum, but the panel is putting out the same amount of light and your brightness slider has not moved. If your screen is painfully bright against a dark room, warming it is a partial fix at best. CircadianShield adds a software dimming layer that goes below the operating system's own brightness floor, and the reasoning is in the monitor brightness and eye strain guide.

Flicker. Most displays dim their backlight with pulse-width modulation, switching the light fully on and off many times a second. A sensitive minority reacts to that pulsing with headaches and eye strain, and it usually gets worse as you turn brightness down, because the pulse gets deeper. No color filter touches this. PWM happens in the backlight driver, a layer below anything f.lux, Night Light, or CircadianShield can reach.

Is f.lux better than Windows Night Light?

They solve the same problem at different levels of control, and the right answer depends on how much you want to fiddle.

Microsoft's Night Light is built in. You reach it from the Network or Speakers icon on the taskbar, or through Settings, then System, then Display. It gives you one Strength slider to set the intensity of the blue light reduction, and a Schedule night light toggle where you pick either sunset to sunrise or fixed hours. Microsoft's own description is that your display will show warmer colors at night that are easier on your eyes. That is the entire feature, and for a lot of people that is the whole job done with nothing installed.

f.lux gives you more handles: named presets down to 1200K, separate daytime and nighttime targets rather than one slider, a temporary disable for color work, movie mode, and Philips Hue control. The trade is a download and a few minutes of setup.

If you want the fuller field, including the open source options and the Mac equivalents, CircadianShield keeps a running comparison at f.lux alternatives in 2026. On a Mac the equivalent question is f.lux against Night Shift, which is covered in Iris vs f.lux vs CircadianShield.

How to set up the f.lux screen filter, and how to switch it off

Setup takes about two minutes, and the step people skip is the one that makes the filter appear broken. CircadianShield recommends running through all six of these rather than just installing and hoping.

1. Download it from justgetflux.com. Use the developer's own site rather than a download portal. Free for macOS, free on Windows for personal use, free for Linux.
2. Set your location. f.lux schedules itself from sunset and sunrise where you actually are. A wrong location is the most common reason the filter seems to do nothing at the time you expect it to.
3. Tell it what indoor lighting you have. The setup asks, because the point is to match the screen to the room you are sitting in. Most write-ups leave this step out entirely.
4. Pick a nighttime preset. Start at Incandescent, 2700K. If the screen still feels harsh at midnight, step down to Candle at 1900K or Ember at 1200K. If it feels too orange to work in, go up to Halogen at 3400K.
5. Judge it after dark, not before. The schedule has nothing to do at three in the afternoon. Look at it an hour after sunset before deciding whether it is working.
6. Learn the disable, not the uninstall. f.lux has a setting that returns your screen to its normal settings for one hour, meant for color-sensitive work like photo editing, and a movie mode that runs two and a half hours with warmer tones while preserving sky colors. On Windows the documented hotkeys Alt-PgDn and Alt-PgUp dim the desktop monitor, and hotkeys can be turned off entirely in the Options menu.

When the f.lux screen filter looks like it is doing nothing

Work through the f.lux FAQ's own three checks first, in this order. Has sunset actually passed at your location? Is your location setting correct? And is your nighttime setting still on Daylight, which would leave the screen unchanged by design?

If all three are right and the screen still looks the same, three other causes account for most of the remaining cases. Content shown in HDR travels a path that ordinary color tables do not affect. Another color tool, a monitor calibration utility, or a GPU control panel can rewrite the same tables after f.lux does. And a monitor running a strong vivid or game picture mode of its own will apply that on top of whatever it receives, which can swamp the change. Set the picture mode back to standard and retest before concluding the software is broken.

On a desk with more than one screen there is a further wrinkle, because external monitors are often driven differently from a built-in panel and a filter reaching one screen does not guarantee it reaches the other. CircadianShield wrote up a five-step photo check for that case on the f.lux on your monitor page.

Where CircadianShield goes further than the f.lux screen filter

f.lux is good software that has been free since February 2009, and CircadianShield is not going to argue it is bad. The differences below are the specific places a color-only sunset ramp runs out. The ranking is by circadian effectiveness using CircadianShield's own methodology, not an independent test, and two rows go the other way on purpose.

One platform note that catches people out: f.lux is not on the iOS App Store. Apple has not allowed it there because of its use of restricted developer tools, and the iOS version requires a jailbroken device. Android has a preview version. So "f.lux on every screen I own" is not actually on the table, which matters if most of your evening screen time happens on a phone.

On privacy, CircadianShield's screen filtering and health scoring run entirely on your machine, and no screen content or personal data ever leaves it. The app does send optional anonymous usage statistics, simple event counts, with an off switch in Settings. No account is required. The full list of what CircadianShield does and does not do is on the brand facts page.

f.lux screen filter: frequently asked questions

What is the f.lux screen filter?

f.lux is a small free program that changes the color temperature of your computer's display according to your location and the time of day. Its own description is that it makes the color of your display adapt to the time of day, warm at night and like sunlight during the day. You tell it where you live and what kind of indoor lighting you have, and after that it runs on a sunset to sunrise schedule without being asked. The screen filter is a color change only. It rewrites the color values on their way to the display, so the panel, the backlight, and the monitor's own menu settings are never touched.

Is f.lux actually good for your eyes?

For evening comfort, a warmer screen is a reasonable thing to try, and many people find a 2700K or 1900K screen easier to look at in a dark room than a 6500K one. What f.lux cannot do is address the two causes of screen discomfort that have nothing to do with color. It does not lower brightness, which its own FAQ states plainly, and a screen that is far brighter than the room is a common source of evening strain. It also cannot touch backlight flicker from pulse-width modulation, because that happens in the backlight driver below any software. CircadianShield is honest about this because it works at the same color layer: if your discomfort tracks with how bright the screen is rather than what time it is, a color filter is not the fix you need.

Is f.lux software free?

Yes. f.lux is free for macOS, free on Windows for personal use, and free for Linux, downloaded from justgetflux.com. Corporate Edition and site licenses are sold separately, so a company rollout is not automatically covered by the free download. The program was released in February 2009 by Michael Herf and Lorna Herf and is developed by F.lux Software LLC. For comparison, CircadianShield is paid at $47 one-time for up to 2 computers with a 7-day free trial on Mac and Windows, card required, so the question is not free against paid, it is whether the extra control is worth $47 to you.

Does f.lux actually work?

It does what it says it does. Set it up and your screen visibly warms after sunset, which you can see for yourself the first evening. Whether that translates into better sleep is a separate question with a less comfortable answer. Wikipedia's summary of the research states that f.lux itself has not been scientifically tested to determine its efficacy. It cites a 1996 study linking reduced exposure to bright blue light at night with increased melatonin secretion, a 2018 study finding that changing the spectral composition of self-luminous displays without changing their brightness settings may be insufficient for preventing impacts on melatonin suppression, and a 2021 study of Apple's Night Shift in 167 college undergraduates that found no effect on sleep latency, duration, efficiency, or wake after sleep onset. The pattern in those results is that spectrum alone, at whatever brightness you happen to be running, is a weak lever, which is why CircadianShield calibrates to Melanopic EDI rather than to Kelvin.

Is f.lux better than Windows Night Light?

f.lux gives you more control and Night Light gives you less to install. Microsoft's Night Light is one strength slider in Settings under System and then Display, with a schedule you can set to sunset to sunrise or to fixed hours. f.lux adds named presets down to 1200K, separate daytime and nighttime targets, a one-hour disable for color-sensitive work, a movie mode, and Philips Hue control. If you want a warmer evening screen with no download, Night Light is already there and does the job. If you want to choose the exact warmth and turn it off on demand, f.lux is the better tool. Neither one dims the screen or affects flicker, which is where CircadianShield picks up.

How do you turn off the f.lux screen filter?

Use the disable option in the f.lux menu rather than quitting the program. Its FAQ describes a setting that returns your screen to its normal settings for one hour at a time, meant for color-sensitive work such as photo editing, and a movie mode that runs two and a half hours with warmer tones while preserving sky colors. On Windows there are documented hotkeys, Alt-PgDn and Alt-PgUp, for dimming the desktop monitor, and hotkeys can be switched off entirely in the Options menu. Temporary disable is the better habit than uninstalling, because the filter comes back on its own afterwards.

Does f.lux make your screen dimmer?

No, and its own FAQ says so: f.lux changes the color temperature of your display and does not adjust brightness. A warmed screen frequently reads as dimmer, because your eye is less sensitive at the red end of the spectrum, but the panel is emitting the same amount of light and your brightness slider is exactly where you left it. This matters if your real complaint is a screen that is painfully bright in a dark room. Warming it is a partial fix at best, and software that dims the image below the operating system brightness floor, which is what CircadianShield adds, is the part that addresses it directly.

Can you use f.lux for design or photo work?

Not while it is active. The f.lux FAQ is direct about this: it is not designed for use during advanced color work, but it is fine for layout or HTML. The intended workflow is to use the one-hour disable while you are grading a photo or picking brand colors, then let the filter come back afterwards. If you do color-critical work most evenings, treat any screen filter, f.lux or CircadianShield, as something you toggle rather than something you leave running.

Why is my screen not changing color with f.lux running?

Start with the three checks in the f.lux FAQ itself: confirm sunset has actually passed at your location, confirm your location setting is correct, and confirm your nighttime setting is not still set to Daylight. If all three are right and the screen still looks untouched, the usual remaining causes are content displayed in HDR, which travels a path ordinary color tables do not affect, another color tool or GPU control panel rewriting the same tables afterwards, or a monitor applying a vivid or game picture mode of its own on top of what it receives.

What does CircadianShield do that the f.lux screen filter does not?

Four things. It calibrates to Melanopic EDI under CIE S 026:2018 instead of to Kelvin alone, so the target is the biological response rather than a color number. It runs 11 twilight phases derived from real solar elevation using Meeus astronomical algorithms rather than a single sunset ramp, and adds a morning blue boost at civil dawn. It dims the image below the operating system brightness floor and includes PWM comfort modes. And it gives each connected display its own setting, up to 8 monitors. f.lux wins on two counts worth naming: it is free, and its 1200K Ember preset goes warmer than CircadianShield's 1800K floor. CircadianShield runs on macOS 14 and later and Windows 10 and later at $47 one-time for up to 2 computers.