The short version
Your body has a clock. The clock takes its main cue from the light hitting your eyes. If that light disagrees with the time on the clock — say, office-cool LEDs at 10pm — the clock drifts. Drifted clocks make for bad sleep, low energy, and worse moods.
Why it matters
Until about 1880 the only light humans saw after dusk was fire — warm, dim, almost no blue. Today the average evening living room is brighter and bluer than noon on an overcast day. The clock notices.
Reading
If you want the deep dive, Satchin Panda's The Circadian Code is
the most accessible primer. The melanopsin literature is where things
get interesting.
What Rhythm does about it
Rhythm makes the light in your home behave the way light outside behaves. Cooler and brighter at solar noon, warmer and dimmer toward dusk, almost entirely off-spectrum after sunset. The hub does this continuously and quietly — you mostly stop noticing it after a week.