Art De Vany examines Sun Exposure Variation, Modern and Ancient:
One of the odd things about modern life is that we have dampened the routine exposure we get to sunlight through our use of shelter, clothing, and (now) avoidance of sun and sunblocks. This lessened daily exposure to sun causes our systems to down regulate vitamin D and melatonin production. Both are known to be protective of cancer.Then we take the family to Cancun and lay in the sun for a week. Or, we hike in bright sun and thin air. Or, spend a week on Palm Springs golf courses. Or, run a marathon or two. This is quite a shock to a body that is adapted to low sun exposure. This kind of low-level exposure with intermittent big shocks may paradoxically be far worse than a higher level of more routine exposure. The protection thought to be provided by routine avoidance sets us up for greater shock and damage when we take the plunge of high exposure. This highly discontinuous variation in sun exposure is far from a natural rhythm of more constrained variation that would have been true in evolutionary times and even in the recent past.
Constrained variation dampens adaptive capacity. Discontinuous variation then becomes a potentially damaging shock to a system that has reduced adaptive capacity. I have made this argument before about exercise and eating and even alluded to the need for temperature variations too. All challenge and expand our adaptive capacity and are protective.