circa
why follow the sun?
your body clock is set by light. a receptor in your retina called melanopsin, tuned to blue-green light around 480 nm, tells your brain what time it is. bright, blue-rich light during the day keeps you alert and anchors your circadian rhythm. the same light in the evening suppresses melatonin and pushes sleep later.
screens are the brightest, bluest thing most of us look at after sunset, and they are calibrated to 6500 K, bluer than noon daylight. circa removes that evening signal in the two ways the science says matter: it warms the color, less melanopic stimulus, and it lowers the light, because suppression follows intensity, not just color. and it deliberately leaves your days untouched: bright days make your clock more resilient to evening light.
a filter is not darkness. the last stretch before bed still belongs to dim rooms and closed lids. circa just stops your screen from arguing with your biology in the hours before that.
how it works
every five seconds it computes the sun's elevation over your location (pure astronomy, no weather api) and blends your screen between your day and night color temperatures. the screen is always drifting with the sun, a gentle slope from sunrise to peak to sunset, then the main dive across civil twilight, +6° to −6°, into full night. changes apply through the display's gamma table, so screenshots and screen shares stay normal for everyone else. location comes from the system or your ip, rounded coarse; nothing else leaves your machine. free, for mac and windows, mit licensed.
the research
- evening room light suppresses melatonin in nearly everyone, before bed, at ordinary indoor levels. Gooley et al., J Clin Endocrinol Metab, 2011
- the circadian system is most sensitive to short-wavelength (blue) light. Lockley, Brainard, Czeisler, 2003
- melatonin suppression follows a dose-response with light intensity, which is why circa dims at night instead of only tinting. Zeitzer et al., J Physiol, 2000
- your recent light history matters: brighter days make you less sensitive to light at night. Chang, Scheer, Czeisler, J Physiol, 2011
- reading a light-emitting device before bed delays melatonin, sleep, and next-morning alertness compared with a printed book. Chang et al., PNAS, 2015
- the american medical association recommends minimizing light at night, including from electronic media. AMA Council on Science and Public Health, 2012
the f.lux team maintains a longer, excellent research bibliography that circa gladly stands on.
isn't this built into my os?
partly. night shift on mac, night light on windows: every major os ships the color half of this idea, and that is genuinely good news, it means the science went mainstream. but the built-ins do one intervention out of four.
they tint on a schedule, usually gently, and stop there. they never dim, even though melatonin suppression follows light intensity as much as color. they flip at sunset with a quick fade instead of following the sun's real descent. they can't go near candlelight depth. and none of them touch flicker: no pwm-safe backlight pinning, no dimming below the hardware minimum.
circa covers all four, shows you the live solar data it's acting on, and is open source. one note: run only one of these at a time. two systems steering the same display fight each other, so turn night shift or night light off when circa is on.
how circa is different from f.lux
f.lux is the fifteen-year veteran of this idea, and it is genuinely good: deep options, per-app exclusions, presets, hotkeys, and real color-science pedigree. it is also closed source, and its interface shows its age.
circa is what you get rebuilding the idea in 2026: open source with a small, readable codebase, a design that feels native today, and opinionated defaults instead of a settings panel. the curve is tied to the sun's actual elevation through civil twilight, night dimming rides the same curve automatically, and flicker-free dimming is built in rather than left to a separate paid app. if you want granular control, f.lux remains the right tool. if you want it calm and automatic, that is circa.
how circa is different from iris
iris is the feature-maximal, paid option: dozens of modes, per-app rules, and, to its credit, the pwm flicker protection idea that circa also implements. it costs money and packs a dashboard's worth of switches.
circa takes the two interventions that carry the evidence, sun-synced color with night dimming and pwm-safe backlight pinning, and ships them free, open source, and battery-aware: flicker-free mode suspends itself on battery because a pinned backlight costs real runtime, something worth being honest about.