Tanning happens when our skin is exposed to UV-light. When UV-light enters our skin, it produces a pigment called melanin, wich gives our skin that golden brown look. The more melanin, the darker the skin.

So what is UV-light? Actually, all light is nothing but electromagnetic waves. These waves can be anywhere from very short to veeeery long. If you would try to put it schematically, the electromagnetic spectrum looks something like this:

UV

You see, only a small part of this spectrum can bee seen by the human eye. We perceive these wavelengths as different colors ranging from violet (400 nm) to deep red (780 nm). Electromagnetic waves with a lower frequency (thus a longer wavelength) are called infrared. Waves with a higher frequency (thus a shorter wavelength) we call UV-light.
UV-light can also be divided into categories: UV-A, UV-B and UV-C. In tanning beds only UV-A and UV-B are used.

Electro

UV-A is mostly responsible for tanning: the longer waves penetrate the skin fairly deep and brown our skin by causing the melanin in this deeper layers to oxidize. This provides a rather fast tan. (Fun fact: did you know UV-A is also known as blacklight?) A tan from UV-A rays is fairly safe as sunburn rarely occurs under the influence of UV-A, but provides little protection against sunburn.

UV-B rays are shorter, thus penetrating the skin more superficially but with higher energy. UV-B does not oxidize melanin, but boosts the production of melanin in the skin. This provides a longer lasting tan and also protects your skin against future sunburn. While UV-B protects, it is at the same time the most likely to cause skin damage. The trick with UV-B is: gradually build up. It’s much like training your muscles: increase exposure a little bit at a time and your body will adjust perfectly. UV-B is also responsible for vitamin D production in the skin. (Why is this important?)

Good to know: most tanning beds produce almost exclusively UV-A, to avoid the risk of sunburn and ensure a fast tan. While the risks of over-exposure are higher, UV-B does have the most benefits for your skin and your health.