What's happened is that during the night a whole series of internal events transpired which ultimately led to "erythema" (reddening of the skin). First, in an attempt to repair the cells that were damaged by the sun's ultra-violet rays, vessels widen in order to rush blood to the surface of the skin. This process, called "vasodilation," isn't the actual sunburn, but just the body's initial defense reaction. Next, minute blood vessels called capillaries break down and slowly leak blood. As this is occurring, the skin, after its exposure to the sun, manufactures an increased amount of melanin, the pigment that makes us appear darker. In the body's next line of defense, a fatty acid compound called "prostaglandins" is released, which plays a role in delaying the effects of sunburn.
The whole process usually takes between 15 and 24 hours, depending on the amount of melanin a person already has in their skin hence, the delayed reaction to sunburn. Of course, after this, your skin will start to peel yet another of the body's attempts to heal itself. Peeling is the skin's effort to thicken up in preparation for further sun exposure. The skin thickens and darkens with each sun exposure, although some individuals, lacking the ability to tan, suffer sunburns with each sun exposure. If all this sounds like it's taking a heavy toll on your skin, it is. As dermatologists everywhere will tell you, tanned skin is damaged skin.