Time in the sun comes with a long list of benefits. It's a great source of vitamin D, and it can leave you feeling energized. But it also does something a little strange to us. Spend long enough out in the sunlight and your skin gets darker — yet your hair, at the same time, tends to get lighter. Why do the same rays do opposite things to two parts of the same body?
Here's the secret: it comes down to whether the cells involved are alive or dead.
What gives skin and hair their colour?
Before we get to the sun, we have to talk about melanin. Melanin is the pigment found in both hair and skin, and it's the reason for any darker colour. The special cells in our skin, called melanocytes, make two different kinds of melanin: eumelanin, which gives a brown or black colour, and pheomelanin, which gives a yellowish-red colour. The amount and concentration of each type is what decides the final colour of our skin and hair.
What the sun actually does
We all know sunlight comes with harmful UV rays. Over prolonged exposure, those rays start to destroy melanin. That matters because melanin works like a protective layer for us against those very UV rays. The degradation of melanin happens in both the skin and the hair — but here's where the two parts of the body behave very differently.
Skin cells are alive — and they fight back
Skin cells are alive, so they can respond to the harmful UV rays. They signal the body to ramp up melanin production to provide more protection. As a result, there's an overcrowding of melanin on the upper layer of the skin, which makes it darker. Over time, continuous exposure builds up into what we call a tan.
Now here's the part I find fascinating about all this.
Hair cells are not — and they can't
Hair cells, on the other hand, are basically dead cells containing water, lipids, proteins, and little else. That means they can't respond to protect themselves, so the damage can't be reversed the way it can in skin. Until the next hair strand grows in, that lightened colour is simply going to stay put. No emergency melanin production, no defence — just the marks of time spent in the sun.
The difference is life itself
That's the whole story, really. Whether the sun makes something darker or lighter comes down to whether the cells involved are alive or dead. Alive, and they fight back by producing more protective pigment. Dead, and they just carry the damage until they're replaced. It's a neat little split between two parts of the body that look so similar but behave so differently.
The appearance of hair and skin may seem alike, but the way each responds to a stress like sunlight is completely different because of what's actually happening underneath. Understand the cells, and the mysterious opposite behaviour suddenly makes perfect sense.
A word of caution
One more thing worth remembering: overexposure to harmful UV rays is said to increase the risk of skin cancer. So while the sun has plenty of benefits, it's worth protecting your skin along the way — a good sunscreen or other protective layer can make a real difference.
That's it for today's curious question. Until next time, let curiosity do its work.