Red Screen Filter for Mac: Deep Red

Most people searching for a red screen filter land on Night Shift, crank the slider to max, and assume they're covered. If what they wanted was a red screen, they are not there yet, because Night Shift's warmest setting is the color of a warm incandescent bulb, which is a warm white and nowhere near red.

A screen only starts to look red past deep amber and deep orange, and Sundown's warmth control reaches a deep red. The step from Night Shift's warmest to Sundown's is a big one, because it takes you from a warm white screen to a deep red one.

What Is a Red Screen Filter, and Why Would You Want One?

A red screen filter shifts your display's color temperature toward the red end of the visible spectrum, so the screen looks amber and then red instead of blue-white, and the warmer the setting, the redder the screen. Night Shift stops at warm white, and Accessibility Color Filters can apply a red tint but do not offer a colour temperature setting.

Your retina contains specialized cells called intrinsically photosensitive retinal ganglion cells (ipRGCs). They express melanopsin, a photopigment most sensitive to blue light at around 480nm, and they signal your suprachiasmatic nucleus, the body's master clock. In laboratory studies, short-wavelength light suppressed melatonin and shifted the clock more than longer wavelengths did.

Melanopsin is far less sensitive to long-wavelength red light (620 to 700nm) than to blue. A randomized crossover trial in PNAS found that evening reading on a light-emitting eReader, compared with a printed book, pushed dim-light melatonin onset more than 1.5 hours later (Chang et al., 2015). That study did not test a red or warmer screen setting, so it tells you what an unfiltered screen did and says nothing about a red one.

"Warm" and "red" are different things. A warm white screen is the color of a household bulb, a warmer screen turns deep amber like candlelight, and only past deep orange does the screen actually look red, which is why how far a filter goes matters.

The Red Spectrum: Why Color Temperature Numbers Matter

Color temperature is measured on a scale where lower numbers mean warmer, redder light and higher numbers mean cooler, bluer light. Your Mac's default display runs at ordinary daytime white, with a bluish cast.

Filter apps and built-in tools are all sold as "blue light filters," but they stop at very different temperatures. Here is how those temperatures look:

Look at the jump between Night Shift's warmest and Sundown's, which takes the screen from a warm white to a deep red. Night Shift users sitting at its warmest setting are still looking at a warm white screen, yet both get marketed under the same "blue light filter" umbrella. Sundown's Blue Light Calculator shows the color temperature each setting gives your screen, side by side.

You can see the gap for yourself: a warm white looks like a warm living room, and a deep red looks like campfire embers.

What Are the Built-in Mac Red Screen Options, and Do They Work?

Apple provides Night Shift and Accessibility Color Filters.

Night Shift adjusts color temperature on Apple's Sunset to Sunrise schedule, and its maximum warmth is that of a household bulb, which is a warm white and still short of red. For context, the Harvard research on blue light and melatonin compared pure 460nm blue light with 555nm green light in the lab, and it did not test Night Shift. A 2021 trial that did test Night Shift, on iPhones, found no significant difference in sleep measured by wrist-worn sensors between Night Shift on, Night Shift off, and no phone in the hour before bed (Duraccio et al., 2021).

Accessibility Color Filters (System Settings → Accessibility → Display → Color Filters) can apply a red tint. The trouble is that there is no granular temperature control, no scheduling, and no way to set a specific warmth, so you get a blanket red overlay. It is a blunt instrument designed for color vision needs, and evening use was never its job.

Reducing brightness doesn't change the color. A dim daylight-white screen emits less light of every color, blue included, but the mix is identical, so it still looks blue-white. Some MacBook displays dim the backlight with PWM, and whether yours does depends on the model (NotebookCheck found it on the 2020 MacBook Air's Retina display). PWM switches the backlight on and off far faster than you can see, which we covered in detail in our screen flicker headache guide. External monitors can dim the same way, and PWM Monitor Flicker: Why Your Screen Hurts shows how to test yours.

Red Screen Filter Apps Compared

Four apps compete for this job on macOS, and below is what their specifications show. For a broader comparison including blue light blocking and other features, see our best blue light app for Mac breakdown.

f.lux's regular warmth settings reach further than Night Shift, which gives a deep orange appearance, and it also offers a separate Darkroom mode with a red appearance and inverted colors. For the full head-to-head, see our Sundown vs f.lux comparison.

Iris has had a PWM-related mode. Iris does not publish a macOS build date, and its downloads sit behind JavaScript, so we could not verify a recent Iris macOS release.

Sundown reaches its warmest, a deep red of the kind where your screen looks like it is lit by a single ember. It also includes a Flicker-Free Mode, which holds the backlight at full power and dims through the gamma table instead. If flicker is on your mind, our guide to screen flicker and headaches covers what the research does and does not show.

If you are coming from f.lux, see our guide to switching from f.lux to Sundown. What a deep red setting changes is how the screen looks, which is not a sleep result, since none of the studies on our science page tested Sundown.

How to Set Up a Deep Red Screen Filter on Mac Tonight

Step 1: Download Sundown from trysundown.com. Every plan except lifetime starts with a 7-day free trial.

Step 2: Open the DMG, drag Sundown to Applications and launch it, and a small icon appears in your menu bar.

Step 3: Click the menu bar icon and drag the Warmth slider to the right, toward red; at the far right it reaches a deep red, the warmest step on the ladder. If the full red feels too aggressive on the first night, stop short of the far right and work your way there.

Step 4: Decide when it goes on. The simplest habit is to pick your warmth from the menu bar when your evening starts, and you can also build a custom schedule by time of day. If your own sleep phase runs later than most, our guide to delayed sleep phase and your Mac screen covers picking a different anchor time.

Step 5: Enable Flicker-Free Mode if your MacBook display dims with PWM and you want dimming that does not rely on the backlight's PWM. It holds the backlight at full power and dims through the gamma table instead, and our guide to backlight flicker and dithering explains the difference between the two.

That's all there is to it.

The Science: What Red Light Does and Doesn't Do for Sleep

The study most often cited is from Harvard Medical School (Lockley, Brainard and Czeisler, 2003). Researchers exposed people to 6.5 hours of monochromatic blue light (460nm) or green light (555nm) at equal photon density, and the blue light caused about twice the melatonin suppression and twice the delay in the body clock. That was pure laboratory light and not a screen, and it did not test red light. We covered what this means for MacBook users in our guide to reducing blue light on MacBook Pro.

A PNAS study found that participants who read on a light-emitting eReader before bed took longer to fall asleep, had reduced melatonin secretion, and were less alert the next morning compared to when they read printed books. The eReader was used without any color filter, and the study did not test a warmer setting.

In the National Sleep Foundation's 2022 Sleep in America Poll, 58% of US adults said they often look at screens in the hour before bed.

Melanopsin is far less sensitive to red light at 620nm and above than to blue, which is why red is where screen filters end up, but that reasoning is not a trial result. Whether a red screen improves sleep has not been shown: trials of blue-blocking lenses are mixed (see our science page). What a warmer setting reliably changes is how the screen looks, with Night Shift's warmest a warm white, f.lux's a deep orange, and Sundown's a deep red.

FAQ

What does a red screen filter do?
A red screen filter shifts your display's color output toward the red end of the visible spectrum (620 to 700nm), so white areas look amber and then red, and the warmer the setting, the redder the screen. The research on blue light and melatonin comes from pure-color laboratory light and from unfiltered screens, and none of it tested red screen settings; see our science page.

Is a red screen filter better than Night Shift?
It goes much further. Night Shift maxes out at a warm white, while a deep red filter goes all the way to red, so you are comparing a warm screen with a red one. No trial we cite compared the two for sleep.

Does a red screen filter affect image quality?
While it is active, yes, because everything appears red-tinted and colors are not accurate. Turn it off for photo editing, video color grading, or any color-critical work. For evening reading and browsing, color accuracy matters less.

Can I make my Mac screen completely red?
With Sundown at its warmest setting, your screen displays a deep red tint. At that setting the display looks lit by firelight, still readable and usable, but unmistakably red.

Is there a free red screen filter for Mac?
Night Shift is free but limited to its warmest (a warm white, not red). f.lux is free; its regular warmth settings reach further (deep orange), and its separate Darkroom mode gives a red appearance with inverted colors. Apple's Accessibility Color Filters also include a free Color Tint option that can give the screen a red appearance, without a colour temperature setting. For deep red at Sundown's warmest setting, plus a mode that keeps the backlight at full power, Sundown offers a 7-day free trial, then costs $4.99/month or $39/year. A $79 lifetime license is also available. For a full feature comparison, see our Night Shift vs f.lux analysis.

Your Mac's screen is a daylight-white engine pointed at your face every evening. Night Shift warms it to a warm white and f.lux pushes it to a deep orange, while Sundown turns it a deep red. Start your 7-day free trial at trysundown.com.

Further reading

• Color temperature explained: what the warmth settings in this post mean on your screen.
• A Mac evening routine by warmth: when to move from warm white to red.
• Sundown vs Night Shift: why Night Shift stays a warm white.