sunlight passing through the atmosphere emits primarily IR and visible spectrum light for the first 3 hours after sunrise. then IR, visible, and UVa for the next 3. then IR, visible, UVa and UVb for the next 3.

if you get exposure in the first two periods, it sets up mitochondrial conditions that will maximize the positive effects of UVb in the third period. converting cholesterol into vitD, a host of other hormones, and the production of Eumelanin which enables you to handle more sunlight exposure. it builds a solar callous. or what we whities call a tan. being electrically grounded (skin on earth) also diminishes the burn effect.