A tan is nature's gift to a healthy beautiful you.
Your skin is your largest, heaviest organ that is the bodies outer covering to protect against injury, infection, heat, cold and store water, fat and vitamins, so it's pretty important to look after it. In short your skin adjusts to it's surroundings to help protect you. e.g.: your sweat is to cool you down and excrete toxins, a tan is to protect you from Ultra Violet Light (UV). Your skin is rejuvenated about every four weeks which is why a tan (natural or fake) doesn't last.
The skin is divided into three layers, the epidermis (or outer layer) which produces the suntan; the dermis (or middle layer) which contains collagen and other materials vital to the skin's strength, it's ability to repair itself and fight off infections; and the subcutaneous (or bottom layer) which acts as an insulator, a food reserve and also binds the skin to the body.
The tanning process occurs in the epidermis. About 5% of the skin cells here at the base of the epidermis are called melanocytes. On exposure to UVB, melanocytes produce melanin, the pigment responsible for a sun tan. Melanin is made from an amino acid called tyrosine. The melanin (pigment) travels up through the epidermis and is absorbed by other skin cells. When exposed to UVA, the melanin oxidizes or darkens, turning the pigment brown. This creates a protective barrier from UV penetrating the deeper, more sensitive layers of the skin. This whole tanning process is the body's own natural defence against sunburn and skin damage.
Warning: Don't Try To Get Too Much of a Good Thing
Exposing your skin to UV for too long or too often (trying to get too much of a good thing) may damage your skin and increase your risk to skin cancer, further it does not get you a darker tan quicker.
Exposing for too long at one time results in sunburn which is damage to your skin. As it takes time for the skin to react to sunlight you will not notice your skin reddening until well after the damage has begun. Especially outdoors in NZ with our harsh sunlight, add in a light breeze to keep you cool and it's very easy to burn and very hard to know how long is enough but not too much sunlight. This is when you need to wrap up, seek shade or use sunscreen. Never sunburn.
Exposing too often is equally overexposure to UV and should also be avoided. This causes skin ageing or photo ageing and increased risk of skin cancer.
I hear you - why tan at all then - because the body is a complex thing and exposure to UV makes us look good, feel good and it's good for us - moderation is the key.