How Does Sunlight Lead to Skin Aging?

Surely we all heard that sun exposure (UV light) is a driver of skin ageing and an effective skincare routine should include a daily sunscreen. But how exactly can something as nice, warm and mood-boosting as sunlight cause the harm? What does UV radiation actually do to our skin that causes wrinkles and elasticity loss?

The main mechanism of damage is the following. UV light activates cell receptors in the epidermis (upper) and dermis (deeper layer) of our skin. This activation happens within 15 min of sun exposure and lasts for at least 2 hours after it.

The activated receptors start accepting distress signals from outside of the cells. It happens within 30 min of sun exposure and lasts for full 24 hours. The signals activate enzymes within the cells, and the enzymes, in turn, start synthesis of special proteins with the function of "cleaning up" a site of skin wound. The "cleaning up" involves destruction of collagen fibers in the skin.

A single UV exposure causes a strong rise in production of the collagen-killing proteins, and their concentration returns to normal only after 48-72 hours after sun exposure.

Skin cells produce new collagen after the old fibers get destroyed. Unfortunately, this repair is not perfect. The "new" collagen matrix is less robust that the "old" one, and this deterioration progresses every time with new sun exposure. Dermatologists call this process "solar scarring". Over time, exposure to UV leads to accumulation of solar scars, and we get photoaged, wrinkled skin.

TL;DR:  Wear sunscreen every day.