Last Updated: May 2026
I used f.lux for seven years. Installed it the week I got my first MacBook in 2016, dragged the slider to the warmest setting, and forgot about it. It turned my screen amber every evening. I assumed that was enough.
Then I started working on blue light blocking glasses and measuring spectral output with a spectrometer. And I realized f.lux at 1200K was still letting 30% of the melatonin-suppressing light through my display. Every single night.
That realization is why Sundown exists. Not because f.lux is bad — it invented this category and deserves permanent credit. But the science has moved past what f.lux offers, the hardware has changed underneath it, and the app stopped keeping up around mid-2023.
This is the honest comparison. What each app does, what it doesn't, and why the gap between 1200K and 500K matters more than the numbers suggest.
Sundown vs f.lux: The Numbers That Matter
Before the context, here's the raw comparison. Every number in this table comes from testing on a MacBook Pro 16-inch (M4 Pro, miniLED) in May 2026.
| Feature | Sundown | f.lux |
|---|---|---|
| Deepest color temperature | 500K | 1200K |
| Blue light blocked at 460nm | 97% | ~70% |
| PWM flicker protection | Yes | No |
| Anti-dithering (Apple Silicon) | Yes | No |
| Gradual transitions | 30s sunset / 20min sunrise | 20s to 60min |
| Presets | 6 gradient presets | 3 (Day / Sunset / Bedtime) |
| Keyboard shortcuts | Configurable (click-to-record) | Fixed |
| Auto-sunset scheduling | Yes (location-based) | Yes |
| App size | 398 KB | ~8 MB |
| CPU usage | Zero | Low (persistent process) |
| Data collection | None | Daily server ping |
| Last major update | May 2026 (active) | Mid-2023 (stale) |
| Price | $4.99/mo, $39/yr, or $79 lifetime | Free |
f.lux is free. Sundown is not. That's a real consideration, and I won't pretend otherwise. The question is whether what Sundown does beyond f.lux is worth anything to you. Let's walk through each difference so you can decide with actual information.
The 1200K vs 500K Gap: Why Depth Changes Everything
At 1200K, your Mac screen turns a warm amber. Like looking through a pint of honey ale held up to a candle. It's pleasant. It's noticeably warmer than Night Shift's 2700K ceiling. And it still transmits roughly 30% of the light at 460nm — the exact wavelength your melanopsin receptors use to tell your brain whether it's daytime.
At 500K, your screen turns deep red. Think dying embers, not candlelight. At this depth, 97% of the 400-500nm range is gone. Your display is emitting almost exclusively long-wavelength red light that melanopsin doesn't respond to.
That 30% gap isn't marginal. Dr. Steven Lockley and colleagues at Harvard Medical School demonstrated in a 2003 study published in the Journal of Clinical Endocrinology & Metabolism that 460nm blue light suppresses melatonin production at twice the rate of 555nm green light. Your circadian system doesn't respond linearly — it responds to threshold crossings. Below a certain blue light exposure, your brain registers "nighttime." Above it, your brain registers "still daytime, delay melatonin."
At 1200K, many people sit right at that threshold. Some nights your brain gets the message. Some nights it doesn't, depending on room lighting, screen brightness, and how long you've been staring. At 500K, you're well below the threshold. Consistently. Every night.
A 2025 study in Nature Scientific Reports confirmed this: unfiltered high-blue-content lighting suppressed melatonin by 42% compared to amber-filtered conditions. Partial filtering produced inconsistent results across participants. Deep filtering produced consistent protection.
That's the core argument for going past 1200K. f.lux helps. But "helps sometimes" and "helps consistently" are different categories when your sleep is at stake.
PWM Flicker: The Problem f.lux Can't See
If you use a MacBook Pro with a miniLED display (any model from 2021 onward), your backlight controls brightness through pulse-width modulation. The LEDs behind your screen don't dim continuously — they switch on and off thousands of times per second. At full brightness, they're on nearly 100% of the time. At 40% brightness, they're off more than they're on.
This happens at roughly 14.8 kHz on MacBook Pro miniLED panels. Fast enough that you can't consciously see it. Slow enough that your visual system still processes it. The result: headaches, eye strain, and a vague visual discomfort that worsens as evening goes on — exactly when you're dimming the screen with a blue light filter.
f.lux adjusts your display's color temperature. That's all it touches. It has no mechanism to modify backlight behavior because it was designed in 2009 when MacBook backlights were uniform LED arrays without local dimming zones. The hardware f.lux was built for no longer exists in Apple's current lineup.
Sundown includes a flicker-free mode that locks your MacBook's hardware backlight at maximum output and dims the screen through gamma table manipulation instead. Your screen looks as dim as you want, but the backlight never cycles off. The strobe stops. I get emails about this feature more than any other — people who spent months thinking they needed new glasses or had developed migraines, when the actual cause was invisible backlight flicker on a dimmed MacBook Pro.
If your evening headaches started around the same time you got a MacBook Pro with miniLED, and they happen most often when your screen brightness is low, this is almost certainly why. And it's something f.lux cannot address regardless of how deep you set the color temperature. For more on this, read our full guide on fixing screen flicker headaches on MacBook.
Apple Silicon Dithering: The Shimmer at Deep Settings
Every Mac with an M-series chip produces temporal dithering — the GPU alternates pixel color values frame-by-frame to simulate colors it can't render exactly. At normal color temperatures, you'd never notice. At deep warm settings like f.lux's 1200K, the limited color palette makes the dithering visible as a crawling, sand-grain shimmer across your entire display.
I noticed it the first week I started testing deep color temperatures during Sundown development. Thought my display was defective. Tested three different MacBooks. Same shimmer on all of them. It's not a defect — it's how Apple Silicon GPUs handle temporal dithering at extreme color values.
f.lux doesn't acknowledge this because f.lux predates Apple Silicon. The codebase was mature before M1 shipped. The shimmer exists whether or not f.lux knows about it.
Sundown communicates directly with the display coprocessor through IOKit to disable temporal dithering. The shimmer stops. Your deep red screen looks clean and stable instead of subtly noisy. It's one of those things you don't realize was bothering you until it's gone.
Privacy: What Leaves Your Mac
f.lux makes a daily outbound connection to its servers. The developers describe this as necessary for "service improvement." The data includes your location coordinates (for sunrise/sunset calculations) and app configuration.
For a utility that needs exactly two inputs to function — your latitude and a clock — daily server communication is architecturally unnecessary. Night Shift calculates the same sunset/sunrise times entirely on-device. So does Sundown.
Sundown sends nothing. No analytics. No telemetry. No server connections of any kind. The app is 398 KB of native macOS code that talks to your display hardware and nothing else. No bundled databases, no tracking frameworks, no network stack. You can verify this yourself — run sudo lsof -i -n -P | grep -i sundown in Terminal. Nothing will appear.
If you chose a Mac partly because of Apple's privacy stance, an app that phones home daily with your location to report screen color preferences is a philosophical inconsistency worth knowing about.
The Update Question: Active vs Stale
f.lux's last substantial macOS update shipped around mid-2023. The app still runs, still shifts your screen color. But it hasn't been updated for macOS Sequoia's display APIs or the macOS 26 design language. No changelog updates. No forum activity suggesting active development.
For most apps, three years without an update is fine. For an app that modifies display hardware — an area Apple changes with nearly every macOS release — it's a reliability question. f.lux works through compatibility layers rather than native integration with current APIs. Sundown shipped its most recent update in May 2026 and is tested on M4 hardware running macOS 26.
What f.lux Still Does Better
Honest comparison means acknowledging where f.lux wins. Two areas.
Price. f.lux is free. Genuinely free, no trial, no freemium upsell. If your budget is zero and you want warmer screen colors after sunset, f.lux delivers that with no financial commitment. Sundown costs $4.99/month, $39/year, or $79 for a lifetime license. The 7-day free trial lets you test everything before paying, but f.lux wins the price comparison outright.
Per-app disable. f.lux has a "movie mode" that temporarily disables filtering for specific applications — useful when you're color grading photos or watching a film where accurate color matters. Sundown doesn't have per-app controls yet (it's planned for v1.3). Right now, disabling Sundown is a global toggle. If you frequently switch between creative work and regular use in the evening, f.lux handles that transition more smoothly today.
Both are real advantages. Neither changes the core filtering depth, flicker protection, or privacy differences. But pretending they don't exist would make this comparison less useful to you.
Who Should Stay With f.lux
f.lux is the right choice if your budget is zero, you don't use a MacBook Pro with miniLED, you don't notice dithering shimmer, and you sleep well with 70% blue light reduction. It's also the better option if per-app color control is critical — photographers and video editors who need accurate colors during evening sessions get more flexibility from f.lux's movie mode than from Sundown's current global toggle.
Who Should Switch to Sundown
Sundown is the right choice if any of these are true for you:
- You still can't sleep well despite using f.lux. The 30% of blue light f.lux lets through at 1200K is enough to delay melatonin onset in people with moderate to high circadian sensitivity. Going to 500K eliminates the variable.
- You get headaches on a dimmed MacBook Pro. That's PWM flicker, not blue light. f.lux can't fix it. Sundown's flicker-free mode can.
- You notice a shimmer or crawling grain on your display at warm settings. That's Apple Silicon temporal dithering. Sundown disables it. f.lux doesn't address it.
- You care about privacy. f.lux pings its servers daily with your location. Sundown makes zero network connections.
- You want an app that's actively maintained for current macOS. Sundown is built for macOS 26 and tested on M4 hardware. f.lux was last updated for a macOS version two generations ago.
If more than one of those applies, the difference will be obvious within the first evening. Start your 7-day free trial and run Sundown at 800K for a night. Then try 500K. Then go back to f.lux at 1200K. The gap speaks for itself.
How to Switch From f.lux to Sundown
Sixty seconds, five steps:
- Download Sundown from trysundown.com. 398 KB. Installs in under two seconds.
- Quit f.lux from its menu bar icon.
- Open Sundown. Pick a gradient preset or set a custom color temperature. Start at 1200K if that's what you're used to, then experiment deeper.
- Enable flicker-free mode if you use a MacBook Pro with miniLED. Enable anti-dithering if you see pixel shimmer at deep settings.
- Uninstall f.lux when you're ready. Drag to Trash. No daemon to remove.
The 7-day trial gives you full access to every feature. Most f.lux switchers settle between 800K and 500K within the first week.
The Science Behind Going Deeper Than 1200K
The case for deep filtering isn't marketing — it's photobiology. Three studies frame why the gap between 1200K and 500K matters physiologically:
Chang et al. (2014), PNAS: This Brigham and Women's Hospital study found screen users took longer to fall asleep, experienced reduced REM, and felt groggier the next morning — even after sleeping the same total hours. Circadian timing shifted by approximately 90 minutes. Partial filtering reduces the shift but doesn't eliminate it reliably.
Gooley et al. (2011), JCEM: Dr. Joshua Gooley at Harvard Medical School showed that two hours of ordinary evening light exposure cut melatonin by roughly half. The critical insight: suppression follows a threshold curve, not a linear one. Below a certain short-wavelength intensity, suppression drops off sharply. This is why going from 70% blocked to 97% blocked produces a much larger perceived effect than the numbers suggest.
Frontiers in Neurology (2025): A systematic review of 10 RCTs covering 266 participants found inconsistent results from blue light blocking — partly because filter quality varied enormously. Interventions using deep filtering showed clear sleep improvements. Shallow filtering showed no effect. Depth matters.
f.lux's 1200K puts you in the zone where outcomes are variable. Sundown's 500K puts you in the zone where outcomes are consistent. That's the functional difference the science supports.
Frequently Asked Questions
Is f.lux safe to use on macOS in 2026?
f.lux runs on macOS Sequoia and macOS 26 through compatibility layers, but it hasn't been natively updated since mid-2023. Users on M3 and M4 MacBooks occasionally report display profile conflicts. It functions, but it's running on inertia rather than active macOS integration. There are no known security concerns — the risk is compatibility degradation, not safety.
Can I run Sundown and f.lux at the same time?
Not recommended. Both modify your display's gamma table. Running them simultaneously produces unpredictable color output because each app overwrites the other's adjustments. If you want to compare, alternate — use f.lux for a few days at its deepest setting, then switch to Sundown. The difference in depth and display stability will be immediately apparent.
Why does Sundown cost money when f.lux is free?
f.lux monetizes through data collection (daily server pings with your location and configuration) and optional donations. Sundown monetizes through direct payment. The features that cost money to develop and maintain — PWM flicker-free mode, anti-dithering, deep color temperature control below 1200K, and ongoing macOS compatibility — are capabilities f.lux doesn't offer. The 7-day free trial lets you test everything before deciding.
What if I only use my Mac during the day?
If you close your laptop before sunset consistently, you probably don't need either app. Blue light during daytime hours is beneficial — it supports alertness, mood, and circadian rhythm synchronization. Blue light filters are specifically for evening and nighttime use, when short-wavelength light disrupts melatonin production.
Does Sundown work with external monitors?
Yes. Sundown's blue light filtering and anti-dithering apply to all displays connected to your Mac. The flicker-free mode specifically targets the MacBook's built-in miniLED backlight, since external monitors have their own backlight controllers that macOS cannot access. Color temperature filtering works across every connected screen.
Is there a free alternative that matches Sundown's 500K depth?
No. The deepest free option is f.lux at 1200K. Night Shift caps at roughly 2700K. No free Mac app reaches below 1200K for blue light filtering. Sundown's 7-day free trial is the lowest-cost way to test clinical-grade filtering depth on macOS.
For a broader look at every option available, read our full f.lux alternatives roundup. For the Night Shift vs f.lux comparison specifically, see our dedicated breakdown. And for the four-app deep dive covering Iris and Circadian Shield alongside Sundown and f.lux, see our best blue light app for Mac guide.