I was thinking that plants do need some darkness too. But so do the fish. Even bright moonlights can keep the fish awake.
Why would you want lights on 24hrs though? Isn't is just a waste of electricity, especially as people wont be around for 24 hrs a day to see the tank
If it's for viewing, why not go with some blue LEDs and rig them to your tank...... it keeps it dark enough for the fish to sleep, but light enough to watch any noctornal fish, such as a pleco! But just in general, I don't leave them on in the night time....