for fruitarians/hygiene theory, sun is a major component. Often, not getting enough sunshine can make an easy excuse of 'failure', even though 'enough' cannot be possibly defined. When you get into the classical frutarian types - not the current popular raw versions of standard low-fat nutritional gurus - but new-age pioneering kooks like Hotema, Krok etc...it gets a bit more complicated and interesting in ways that I think pertain to what you are saying. For them, the fruit part of the diet is actually seen almost like a solvent for purifying the body, which then becomes a vessel for sun energy. This is looked at as a lifetime process, and both exposure and sun gazing are seen as tools for further purifying the body and as sustainable energy. far out. It is thought in this same vein that various ailments linked to exposure are the body bringing existing toxins to the surface, so the chemical factors are linked internally, not to changes in UV etc...and cases of people close to the equator are cited, which starts yet another diet/genetics debate, although the cases in shifting diet/location of the same people shows that this might have some merit. Other than that, I try not to dwell to much on what makes fruit folks tick. I think just looking at SWD one can see how non-essential true nutrition is to mere existence, but how favorably one can capitalize on true proteins and fats for healing and repair. Writing this I keep thinking about William's "THERE IS NO PROTEIN IN PLANTS" marquee, which ironically speaks more about the value of protein. Even with the ridiculous claims about veg protein 'power', and neglecting the specific value and suitability of animal foods, It just becomes so clear that for tribal folks closer to a vegetarian model, that raw proteins and fats are assets, they don't completely dictate all internal processes. Its therefore very possible, that life force cannot be distilled to molecules.
other things I've picked up, which have yet to be comfirmed, revolve around how the body actually assimilates sun energy, or to put in more practical terms how it intakes vitamin D etc.. I have heard a.) this pretty much only takes place when the sun is high between 10am-2pm, the window becomes even more important in winter months, to the point of some places being impossible b.) certain areas of the body are more of a requirement to be exposed than others, even random things like the back of the knee caps and other areas being essential for proper uptake c.) as mentioned in the sunbed thread recently, that bathing and abrasive materials or soaps might negate/wipe off vitamin D production.
Me personally, I try to get as much time as I can stand between 10-2 in short spurts for probably on average 1-2 hours, and don't bother with other times for intentional exposure purposes anyway. Occasionally and usually limited to spring I get burns in the form of redness, and at the most extreme a sun like sickness/nausea, which usually lasts less than 12 hours, and then becomes tan. So far during frutarian I was at my most darkest (and never could even possibly darken before diet change being sickly pale) but at that point I was also really pushing the sun thing, being outside alot of in summer months, and in the winter lived in SOFL and worked outside all day in basically underwear. The only diffence so far I find is on VLC there is so little that can truly hydrate/balance large amounts of exposure (especially with activity). I get the same boredom, but I guess its working for me in this respect, with a goal of short regular exposure.