All three are connected. For example, sunlight and aging suppress immunity, immunity affects health, and melatonin and DHEA affect them all. Skin is such a specialized organ that it has its own immune system. It has been proposed that faulty skin immunity affects the entire immune system. Sunlight can penetrate deep into skin and alter immunity directly, or it can cause changes in dermis and epidermis that provoke immune changes.
Sunlight affects hormones.
It decreases melatonin, norepinephrine, and acetylcholine and increases cortisol, serotonin, GABA, and dopamine. Studies show that both DHEA and melatonin are absorbed by skin when applied topically. No special carriers are needed to get DHEA and melatonin into skin. A properly formulated topical preparation of melatonin and DHEA will contain just enough hormone to benefit skin without providing enough to escape into circulation. It makes sense to apply the hormones directly to the skin if skin protection is the goal, since ingested hormones may end up everywhere but the skin.