I go to bed around 8:30 p.m. every night (as the lights go out on the fish tanks). Every morning I get up around 3:30 a.m. to about 4:30 a.m. and one of the first things I do is turn the fish tank lights on. They are sleeping at the time, but they would be awake by 5 or 6 a.m. anyway.
I don't know about all fishes, but for mine, it seems they go to sleep around 9 to 10 p.m. or so, then get up around 4:30 a.m. or so.
It is an interesting and intriguing subject, though.
You can turn it on whenever you feel like it. They can sleep with the light on, but it is best to give them a day/night cycle if possible. You can set that day/night cycle however you see fit, though, and they are totally fine if you only have the light on while you are viewing them.
In my main planted tank I have my lights on a timer, it doesn't come on till noon and then goes off at 6pm, moonlights from 6-10. I based that around my schedule because that's when I'm most likely to view the tank.
I absolutely agree with you Ken on the turning a room light on first. It seems to slowly adapt the light to their awakening. Kind of like a distant lighting source, instead of a light that can startle them, etc.
I leave a room light on for about 5 minutes, then turn their light on. I don't know if the 5 minutes is benefiting the scenario or not, but it seems to be.
Yes I usually do that every morning is to turn some other lights on first give them say half hour to acclimate then to turn on the tank light... Yes I agree. Thank you for giving me feedback on my "silly" question...
FWIW - I use a timer on my FW tank in my bedroom. When I get up at 6:20am, if it's too dark for me, I turn on the room light. The tank lights don't come on till 1:30pm. No-one in the tank seems to wake up, maybe Zoe (my angel), but nothing too alarming. The lights are on a timer, they know the routine.
My Reef tank has a simulated Night-sunrise-day-sunset-night feature, so I never worry about it.
I agree that fish need a sleep period, and timers are your best friend. IMO, ensure there is some light in the room so the bright lights don't startle your fish (and inverts such as Shrimp - they hate bright sudden lights)