The goals are simple. The controls, even moreso. All you do in Circadia is tap colored circles to send out musical tones that need to hit their white counterpart. Higher tones travel at faster speeds and lower ones move slower. All the waves need to hit the dot at the same time so you'll need to synchronize the timing on your taps. Circadia's beguiling challenge lies in finding that rhythm.
It's clearly an exercise in minimalism. But as the game goes on, an astounding complexity reveals itself. More white dots show up, as do more circles. More surprisingly, Simple Machines' deconstructed rhythm game does more to make me feel like a musician than Rock Band and Guitar Hero did. Running through a level multiple times isn't just trying to clear a challenge; it's also an exercise to try and make things sound right.
Circadia strips away all of the visual trappings of other rhythm games with the end result making you feel like you're living inside a soundscape. It re-tunes your brain a bit, harmonizing it with a cadence that lets you hear the world differently.