Here’s a question for the start of summer: why does exposure to the sun darken our skin but lighten our hair?
First let’s take a look at our skin. Human skin is the body’s largest organ, and acts as a barrier between our inner organs and the outside world. It’s made up of essentially two parts the epidermis and the dermis. The epidermis is the outer section and is comprised of a layer of living cells, topped by a layer of dead cells. The dead cells are the skin we see.
Even though the upper epidermis is just a lot of dead cells, it contains keratin, a tough protein that also makes up our hair and fingernails, Keratin is thicker on the bottoms of our feet and the palms of our hands for added protection against abrasions and other intrusions from the outside world.
Inside the dermis is where all the skin’s functioning equipment is located, These include nerves, sweat glands, hair follicles, blood vessels and special cells called melanocytes, which produce melanin, the material responsible for skin pigmentation, hair and eye color. Most humans have about the same amount of melanocytes, some just produce more melanin than others. Albinos, however, produce no melanin at all.
When our skin gets exposed to sunlight (particularly ultraviolet rays) melanocytes begin producing melanin to help protect the dermis, and keep the skin cells from getting fried. The melanin acts as an absorbing agent. So over time, as exposure to the sun continues, more melanin is produced and subsequently the skin becomes darker.
The hair is a different story. Hair color is also determined by melanin, but hair cells are dead, so sunlight doesn’t initiate melanin production but rather begins to break down the melanin already in the hair, and the hair’s color begins to fade or lighten.
I thought this last part was strange. The pituitary gland is tied to your optic nerve and is sensitive to sunlight. When light enters your eyes, it triggers your pituitary gland to produce a melanocyte-stimulating hormone (MSH) that activates your melanocytes to produce melanin. This means that wearing sunglasses can actually cause sunburn.