Brightness curves

Brightness is the other half of the engine. Rather than ramp on the clock, Rhythm fits a smooth curve to the day and reads the right level off it at every moment. The curve is anchored to the sun, so it stretches in summer and compresses in winter without you touching anything.

The shape

The default curve is a super-Gaussian — a bell with a flattened top. It rises through the morning, sits near full through the middle of the day, and falls through the evening. The peak is pinned to solar noon (the midpoint between sunrise and sunset), not 12:00 on the clock.

brightness

100% |        _______________
     |       /               \
     |      /                 \
     |     /                   \
 20% |____/                     \____
     +--------------------------------
       sunrise     noon      sunset

The floor isn't zero. By default the curve bottoms out at 20% and peaks at 100%, so even at the edges of the day the lights have a sensible minimum instead of guttering out.

Why a plateau, not a bell

A plain bell curve would mean the lights are only at full for a single instant at noon and visibly dimmer an hour either side. That reads as the room slowly breathing. The flattened top — controlled by a shape exponent (default around 6) — holds the lights steady through the productive middle of the day, then rolls off toward dusk.

Mornings and evenings differ

The left and right halves of the curve have independent widths. Out of the box mornings ramp up a touch faster than evenings ramp down — waking a room briskly but letting it wind down gently. Color temperature has its own pair of widths, so the warm-down in the evening can lag the dim-down, which is what you want for sleep.

Stepping along the curve

When you tap a dimmer's up/down rocker, you're not setting an arbitrary percentage — you're stepping along the curve. Each step moves to the brightness (and matching color) the curve would hit a little earlier or later in the day. The default is six steps between the floor and the ceiling. This is why a manual nudge still looks like Rhythm: it lands on a real point of the day, not a flat 50%.

What you can tune

Per light profile, the knobs that shape the curve are:

• Min / max brightness — the floor and ceiling (default 20%–100%)
• Min / max color temperature — the warm and cool bounds (default 1800K–5500K)
• Shape exponent — how flat the plateau is (rounder bell vs. wider tabletop)
• Ramp widths — separate morning/evening steepness for brightness and color
• Dim steps — how many discrete stops a button press moves through (default 6)