I've just installed an LED strip (self adhesive, 12v, cool) on the ceiling of one of my vivs. It's possibly a bit bright but a much more "daylight" colour than a bulb. I favour the natural cycle. My viv is their ecosystem; and in a natural ecosytem they'd have day and night. I imagine they'd come out at dusk and dawn, and hide during the day and middle of the night. That's pretty much what they do in their vivs. The light comes on at 9am and goes off at 5.30pm which means they get the benefit of the natural brightening light at dawn and fade at dusk. I put plenty of shades and hides in my vivs, bark shelters, branches, resin hides, plant pots, plastic plants, rocks, etc, so the residents have plenty of options.
I think virtually all animals need light. Perhaps not direct sun on their skins, but the presence of light, and the day/night cycle plays a part in many physiological functions. I think that vivs should have a cyclical day/night light, whether natural light or artificial. My house is so dim that I use artificial lights in the vivs, cos otherwise I'd never see the residents and they'd never see me!