The answer is dusk and dawn.
Crepuscular animals are active during twilight (latin crepusculum). Twilight is a period after dawn and before dusk, so crepuscular animals want to avoid dusk and dawn. Animals' activity in different periods during the day depends on temperature or other biotic factors. It also depends on predators' activities or local competition.
If mutation is associated with the centromere region, the direct impact would be on the segregating mechanism during cell division process.
