Direct sun wouldn't be needed but everything benefits from some sunlight. Light bulbs do not produce the same effects as sunlight. Incandescents especially give off poor light quality but even full spectrum lights pale in comparison. Sunlight helps regulate our bodies and theirs in various ways so a regular schedule of sunlight and darkness is needed for the best longterm health. That doesn't mean they need a completely unblocked beam of light just some unfiltered light, if possible not through glass- windows block more sunlight than we realize, every day. I would apply the same light requirements that are suggested for using artificial light to increase laying. That is there should be enough light for you to easily read a book or newspaper during the day without turning on the lightbulb.