Darts do have predators. Perhaps they're retiring early because of a leftover instinct to avoid nocturnal predators? Animals that have excellent night vision have very poor color vision, due to many more rods (which detect light) than cones (which detect color) in their eyes. So maybe the frogs are bedding down before the 'sun' goes down so they won't fall prey to animals that can't see their warning colors before it's too late? Just a guess here.