Do you drop temperatures in your tanks and cut feeding six months a year to stimulate winter for your fish? Do you think you should?
Yes, it is probably better to give your fish a more natural light cycle. It is also almost impossible unless you either have a dedicated fish room or a cabinet that you can close around the tank to block outside light.
The only outcome that I can see gaining from this thread is from the articlesDrstrangelove posted that show that some fish do something similar to human sleep, and that it varies according to species, and a bunch of other factors, and that even human sleep is very poorly understood.
I understand that you have very high standards for the quality of life of the fish you keep, and I understand that without knowing any better the best you can do is imitate the fish's natural environment as closely as possible, but I feel that in this case there is no reason to say that for a given fish lack of a proper daily cycle will effect it in any quantitative way.
I don't have a natural sleep cycle, so why should I let my fish have one?
Most of us keep tropical fish. Those fish do not experience the extremes of hot/cold that temperate species do. Instead, they commonly experience wet/dry seasons. I suspect that this dynamic is why we see breeding activity after water changes. Big shifts in water chemistry could be a evolutionarily co-opted signal that the wet season has arrived and it would therefore be advantageous for a species to reproduce at that time. For temperate species, the reproductive cue is often light (photo-period and angle of light), but it's not outside of the realm of possibility it would be temperature regime.
With respect to the more general question posed by this thread, whether how you are keeping your fish is "ok" for your fish, I think it's a pretty easy litmus test. Simply ask, would my fish experience the conditions found in my tank in the wild? If the answer is no, the answer to the more general question becomes obvious.
