You know what happens when the lights on your tank turn on. If the room is really dark when the lights come one, the fish are usually pretty still and will even be quite pale looking, especially if the lights come after a lengthy period of complete darkness. Why? Because they're asleep, but not in the same way you and I sleep.
Nonetheless, when the lights come on, it often takes several minutes for fish to become active. If you spend any amount of time observing your cichlids, you know that even ambient light (e.g., daylight behind closed blinds or overhead lights in the room) is often enough to keep your fish awake and alert. Most aquarium fish will dart about when the tank lights are on, and most cichlids get quite enthusiastic when you approach the tank.
If you keep mbuna cichlids, don't be fooled by what you don't or can't see. When the lights are on and your mbuna aren't visible, chances are they're hiding, not necessarily from each other but from you and any other potential predators. You may not even notice much serious aggression then. However, when the lights are off and they're awake, there are lots of things going on.
If you have lots of rocks for caves and shelter, which is a good idea if you keep mbuna, grab a seat near your tank when the tank lights are off and observe. The interplay is fascinating, even more so with a mixed species tank. Some fish will be excavating the substrate, especially if you have sand. Some will be actively swimming in and out of the cover, perhaps just to see what's going on.
On the other hand, there may also be territorial disputes that result in posturing or, unfortunately, someone may actually be getting beaten to death. That's right. It can happen and it can happen quickly. In fact, you often won't even see it if you have lots of rocks and caves. In a matter of minutes a cichlid could get pinned under some rock and get bludgeoned to death by a tank mate who, the night before, seemed just as docile as a dither fish.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub