In humans, light affects the pituitary gland, as well, and one result is the production of MSH. A funny side-effect of all of this is that wearing sunglasses may make you more susceptible to sunburn!
If you are @Caucasian and you don't have a tan, then the cells in your skin are not primed with melanocytes, and they are therefore not protected from the sun's ultraviolet radiation. You become an easy target for sunburn if you spend too much time in the sun. As anyone who has a sunburn knows, sunburn leaves your skin red and extremely painful. In severe cases, the skin forms blisters. These responses are actually cellular damage caused from UV exposure. The body responds to the damage with increased bloodflow to the dermis' capillary bed in order to bring in cells to repair the damage. The extra blood in the capillaries causes the redness -- if you press on sunburned skin it will turn white and then return to red as the capillaries refill.
You can block or absorb UV with opaque creams like the white zinc oxide cream that you see lifeguards putting on their noses. These are commonly known as sunscreens. The first and most common of the absorption chemicals is @PABA (para-aminobenzoic acid). It absorbs UV-B, one form of ultraviolet light.
All sunscreens are labeled with an @SPF, or Sun Protection Factor. This applies to cream's UV-B blocking ability. The SPF acts like a multiplying factor. If you would normally be OK in the sun for 10 minutes and you apply an SPF 10 sunscreen, you will be OK in the sun for about 100 minutes. In order for the sunscreen to work, however, you have to apply plenty, and it has to stay on. You should apply it about half an hour before going out in the sun (or the water) so it can bind to your skin -- if you don't, then it is very easy for the sunscreen to wash off.
Regardless of the SPF sunscreen you use, however, and regardless of the degree of your tan, moderation is the key. Even with these protective factors, sunlight can still cause a variety of problems from skin drying to skin cancer. Use your judgment with how long you stay out in the sun before taking a break.
