Deep in caves, where there’s no sunlight, is there any sort of day/night cycle that might be perceptible to the organisms within? Something like a consistent change in atmospheric pressure or humidity or something?
Funny you should ask it like this, because there is a famous experiment involving circadian rhythms and caves.
I have seen aquarium coral isolated from sun light close up for the night in late afternoon, when the aquarium lights are still on.
Isn’t that a deleted line from Roy Batty’s death scene?
(Back on topic, it looks like Mexican blind cavefish, at least, lost their circadian rhythm.
I doubt it.