Sun exposure has both a bright side and dark side.

One of the most beneficial effects of the sun is to aid the production in the skin of vitamin D, one of the body's essential vitamins. Psychologically, sunlight seems to give people a lift.

Then there is the sun's darker side -- the link between premature aging and risk of skin cancer with too much sun exposure. Yet no one suggests that people should keep the hours of vampires and never go out in bright daylight.

"It's crazy to tell people to stay out of the sun," says Dr. Stephen I. Katz, head of dermatology at the National Cancer Institute. "You can't tell people to avoid sun totally. The important thing is to educate them to do it right."

Among the recommendations for sun safety:

*Limit sun exposure when possible, but particularly try to minimize exposure from 10 a.m. to 2 p.m., when the rays are most intense.

*Use sunscreen. The higher the SPF -- or sun protection factor -- the more time before sunburn or tanning occurs. Fair-haired, light-skinned people with blue, green or gray eyes are especially vulnerable to sunburn. Swimming and perspiring also increase the need for reapplication. But experts now caution not to overuse sun screens containing para-aminobenzoic acid, or PABA. Recent evidence by Yale School of Medicine chemist Francis P. Gasparro suggests that sunlight may convert PABA to other chemicals that may harm the skin.

*Apply sunscreen at least 30 minutes before going into the sun to allow for good penetration into the skin.

*Wear appropriate clothing, especially if you are going to be in the sun for long periods of time during peak hours. Experts recommend wearing a hat and a light-weight, long-sleeved shirt or pants to protect against the ultraviolet radiation.

*Remember that many surfaces, including sand, snow, concrete and water, can reflect up to 85 percent of the sun's damaging rays. Ultraviolet rays also can penetrate clouds and cause sunburn even on overcast days.

*Avoid the use of tanning salons and sun lamps. Despite many claims to the contrary, studies show that tanning booths and sun lamps emit many of the same rays as the sun and cause sunburn, premature aging and increased risk of skin cancer.