Upon exposure to the sun, the harmful UV rays damage the existing collagen and elastin in your dermis (deeper layer of your skin) which gives your skin elasticity and support. The less collagen and elastin you have, the more likely you are to produce wrinkles and develop sagging skin.
Again, it is important to protect your skin with sunscreen and use products to help boost your collagen production such as TNS Serum (available at my office) which contains 100 growth factors to help boost collagen production. Botox and Ultherapy are also very effective treatments for tightening skin and improving the appearance of lines and wrinkles.