f.lux is a reasonable first step. It shifts your screen color warmer at night and it does that fine. Circadian Shield is for people who found that was not enough, deeper dimming below what the OS allows, PWM comfort modes for flicker-sensitive users, scheduled display control across 11 modes, and 60+ display tools built around circadian science.
Last updated: July 5, 2026
f.lux is free and simple to install. It has been around for over a decade and runs on Mac, Windows, and Linux. If your only need is a warmer screen after sunset, f.lux handles that adequately.
It applies an automatic color temperature shift based on time of day. For many people, that is enough. There is no setup beyond entering your location, and it stays out of your way.
If that is solving your problem, there is no reason to switch. But if you are here, you probably found that it was not.
These are specific, verifiable differences, not marketing adjectives.
Circadian Shield can dim the display below what the operating system allows. f.lux adjusts color temperature but does not extend the dimming range. If your screen still feels too bright at the lowest OS brightness setting with f.lux running, Circadian Shield solves that.
Some displays use pulse-width modulation to control brightness, switching the backlight on and off hundreds of times per second. That invisible pulsing triggers headaches and eye strain in a sensitive minority, and it gets deeper at low brightness. Circadian Shield includes software overlay dimming that lets you hold hardware brightness high, where most panels run at or near full duty cycle with little PWM, and still get a darker screen. f.lux does not address PWM at all.
Here is the honest limit, the part most comparison pages skip: software dimming cannot change how a backlight behaves at a given hardware brightness. If a display pulses even at 100% brightness, no overlay can remove that, because software never touches the backlight driver. In that case only flicker-free hardware fixes it. Not sure if your screen is affected? Run our monitor flicker test, read the PWM flicker and headaches guide, or learn how to verify a flicker-free monitor before you buy.
f.lux operates on a simple preset model: warmer at night, normal during the day. Circadian Shield offers 11 distinct modes, Auto, Movie, Reading, Coding, Presentation, Gaming, Biohacker, Sunglasses, Dark, Custom, and Disabled, that can be scheduled, triggered manually, or assigned per-app.
Beyond color temperature, Circadian Shield bundles a broader toolkit: per-display profiles, screen effects, spotlight dimming, inactivity pause, and keyboard shortcuts. These are bundled in one app rather than requiring separate utilities.
Circadian Shield calibrates its filtering to melanopic equivalent daylight illuminance (EDI), the measure defined in the CIE S 026:2018 standard. Melanopic EDI describes how strongly light stimulates melanopsin, the photopigment in the intrinsically photosensitive retinal ganglion cells (ipRGCs) that signal time of day to your body clock, with peak sensitivity around 480nm. f.lux and the built-in tools set a color temperature in Kelvin, which correlates with melanopic effect but is not a direct measure of it.
The practical difference is that two screens at the same Kelvin value can differ in melanopic content depending on the panel's spectrum. Calibrating to melanopic EDI targets the wavelengths most tied to circadian timing rather than approximating them through warmth alone. This is a design approach meant to support your evening wind-down, not a medical treatment. For the mechanism, see the melanopic EDI explainer and the science behind Circadian Shield.
Circadian Shield includes built-in screen break reminders with flow detection and video call pausing. f.lux does not include a break timer.
Three tools, including the ones built into your OS. This table reflects our own product testing and methodology, not an independent lab review. Circadian Shield is our app, so weigh it accordingly. The f.lux and built-in features shown are the ones documented at the time of writing.
| Feature | Night Shift / Night Light | f.lux | Circadian Shield |
|---|---|---|---|
| Color temperature adjustment | Yes | Yes | Yes |
| Scheduling | Sunset to sunrise | Location based | Solar position, 11 phases |
| Deep screen dimming (below OS minimum) | No | No | Yes |
| Number of display modes | Single warm slider | Presets | 11 modes |
| PWM flicker comfort mode (software overlay) | No | No | Yes |
| Break timer (20-20-20 rule) | No | No | Yes |
| Melanopic EDI calibration (CIE S 026) | No | Color temperature based | Yes (CIE S 026) |
| Per-display control | System-wide | Limited | Per-display (up to 8) |
| Platform | Built into macOS and Windows | Mac, Windows, Linux | Mac and Windows |
| Price | Free | Free | Pro $79/yr or $9.99/mo (14-day free trial) |
No. Circadian Shield starts at $39/year founder pricing with a 14-day free trial that includes full Pro features. f.lux is free, but it only adjusts color temperature. Circadian Shield adds deeper dimming, PWM flicker and eye strain comfort modes, 11 display modes, a break timer, and melanopic EDI calibration.
Yes. Circadian Shield is available on both Mac and Windows.
The main differences: deeper screen dimming below the OS minimum, PWM flicker comfort modes, 11 schedulable display modes, a 20-20-20 break timer with flow detection, and calibration built on melanopic EDI research rather than color warmth alone.
For some people, yes. If a warmer screen color at night is all you need, f.lux handles that. But if you still find your screen too bright, get headaches from display flicker, or want more control over how your display behaves throughout the day, f.lux may not go far enough.
Install Circadian Shield from the download page, then disable or uninstall f.lux. Running both at the same time is not recommended, overlapping color filters can produce unpredictable results.
Yes. Your subscription covers both platforms.
Yes. f.lux runs on macOS 10.15 and later, supports Apple Silicon, and is a free download from justgetflux.com. It lives in the menu bar and shifts color temperature on a sunrise to sunset schedule once you set your location. Circadian Shield is built for the same Mac users who want more than a sunset color shift.
Yes. f.lux has been available since 2009, has tens of millions of users, and only needs the Accessibility permission required to adjust display color. It does not transmit usage data in any documented way. The one thing to watch is running it alongside another color tool like macOS Night Shift or True Tone, since overlapping filters can fight each other.
On macOS in 2026 the main alternatives are Night Shift (built into macOS, a single warm to cool slider on a sunrise and sunset schedule), Iris (cross-platform and paid, with blue light reduction plus break reminders), and Circadian Shield (solar-position scheduling and melanopic EDI calibration rather than color warmth alone).
No. f.lux adjusts your screen's color temperature and does not touch backlight PWM, the rapid on-off pulsing many displays use to dim. What can help is keeping hardware brightness high, where most panels run at or near full duty cycle with little PWM, and dimming the image with a software overlay instead. That is how Circadian Shield's PWM comfort mode works. The honest limit is that no software can remove flicker from a backlight that pulses even at 100% brightness. In that case only flicker-free hardware fixes it. See our flicker test, the PWM and headaches guide, and how to verify a flicker-free monitor.
They overlap more than they differ. Night Shift on Mac and Night Light on Windows are built in and free, with a single warm-to-cool slider on a sunset-to-sunrise schedule. f.lux is also free and adds finer color control, more presets, and cross-platform support including Linux. If you want a bit more control than the built-in toggle, f.lux is the better free pick. If you also need deeper dimming, PWM comfort, or melanopic EDI calibration, neither goes that far, which is where Circadian Shield comes in.
Melanopic equivalent daylight illuminance (EDI) is the measure in the CIE S 026:2018 standard for how strongly light stimulates the melanopsin in your eyes, the signal most tied to circadian timing. f.lux and the built-in tools work in color temperature (Kelvin), which is a reasonable proxy but not the same thing. Circadian Shield calibrates its filtering to melanopic EDI directly. It is designed to support your evening wind-down rather than to treat any condition. More on the melanopic EDI explainer and the science page.
14-day free trial. Full Pro features. No credit card required.