Why do hair and nails grow faster in the summer?

Living in a very nothern climate, this is obvious to me and friends I’ve talked to about it: I need to clip my nails and visit the friendly barber more often during summer. Spmething with temperature and sun, no doubt, but how/why?

More Vitamin D from more sunshine? Do you take a multivitamin pill?

More nutritious fruits and vegetables (hard to pass up that fresh sweet corn, tomatoes, canteloupe)? Do you tend to eat more fresh fruit and veg in the summer?

More fresh air and exercise? Are you an outdoorsy person in the winter, too, or is it only in the summer that you get outside and do stuff?

If you’re a frozen TV dinner-eating couch potato in the winter, and an “eat your spinach” bike-riding fanatic in the summer, then yeah, I wouldn’t be surprised if your hair and nails grow faster in the summer.

In addition to what DDG posted, I wonder how much humidity plays a role. I’ve noticed that since moving to the East Coast, my nails are far less brittle during the summer than the winter. I’m not sure that they grow any faster, but I think they break less frequently. I didn’t notice a seasonal difference when I lived out West where it is much less humid.