It’s midnight, the room is dark, the brightness is at the very bottom of the slider, and the screen is still lighting up the wall behind you. The lowest key press is not the darkest a screen can be. It’s the darkest the backlight can be, and there are ways to keep going. Here they are, cheapest first, with the one caveat that applies to all of them.
Why there’s a floor at all
Every backlight has a bottom, a dimmest its designers allowed, and no two screens put it in the same place. Your slider ends where the lamp ends. On some MacBooks the floor is a genuine whisper; plenty of external monitors idle at what a dark room reads as a reading lamp. If it’s an external monitor that’s too bright at minimum, start with when the answer is no, because that monitor’s own menu almost certainly has knobs (Contrast, an sRGB or Reader mode, an eco setting) that lower its floor for real, for free, before any software trick is needed.
macOS has no setting that goes darker
You may have read that a setting called Reduce White Point does this. It does, on an iPhone or iPad. The Mac never got it. Its System Settings > Accessibility > Display pane covers contrast, transparency, pointer size and color filters, and nothing there lowers brightness. Past the floor, every option on a Mac is an app that darkens the picture itself. Two of them are free: the one this site is for, covered below, and Lunar, whose free version includes what it calls sub-zero dimming.
Darkening the picture is also the caveat. It’s software dimming: the lamp behind the picture keeps burning at whatever level it was already at. The glow of a “black” screen in a dark room doesn’t change, no battery is saved, and contrast drops a little. For a couple of late hours, none of that matters much. I’ve written about what separates darkening the picture from turning down the lamp if you want the full accounting.
What won’t help
Night Shift is the setting everyone reaches for first, and it’s the wrong tool. It changes the color of the light, warming it toward amber; it barely changes the amount. A warm bright screen is still a bright screen. Dark Mode helps a little by painting fewer white pixels, but on most Macs the backlight underneath stays where it was. The untangling of Night Shift, True Tone and brightness is its own post.
Dimming below zero, on purpose
This is the exact situation Brightness Controller’s sub-zero dimming exists for: rooms darker than your monitors can go. It works in Calibrated mode. Switch to Calibrated, turn on “Let the master go below the monitors’ minimum” in the advanced settings, and the master slider’s range extends below the hardware floor. The backlights come down together until the first panel reaches its minimum. From there the app keeps going in software, and every screen still moves together, so no panel sits stuck at its floor while the others keep dimming.
Two design details matter here. It ships switched off, behind the advanced-settings gate, because darkening the picture has real costs and choosing them should be your call. And it stops short of black, always, so no screen can ever go so dark you can’t find your way back to the control that undoes it.
One caution about the other direction
A tempting variant of this problem is forcing the backlight lower than the manufacturer allowed. Don’t chase it. Many panels control brightness by flickering the backlight faster than you can see, and at the very bottom of the range that flicker can slow into visible, headache-grade territory. It’s why the app has a backlight floor setting that refuses to drive a panel into that zone, and does the last stretch of dimming in software instead. Below the floor, the picture is the right thing to darken. The lamp has given what it has.