They vary, not sure what actually triggers spawning though. In the past (before 2005) many start to spawn at dawn - 9am. Nowadays most of mine spawn around 8-9am. But there are occasional "late" spawners that begin after 1pm. They seem to know when it's day time and when it's night. Though I keep my breeding tubs dark (totally covered black tub), they are active during the day but become inactive after dark. Even lighted tubs doesn't make them active at night.
Maybe it is also influenced by how they were raised. I've noticed that fry used to being fed at night, are more active than those that aren't. . . . with extra lighting. But they are basically the same if the lights are turned off.