Does Summer Really Make Your Hair Grow Faster? The Science Behind the Myth

When the days get longer and the sun feels hotter on our skin, we tend to notice everything a little more: the sweat at the back of our neck, the urge to slap on sunscreen, and—oddly enough—the belief that our hair is sprouting like weeds. It’s a claim you hear at barbershops and on social media, but does the science actually back it up?

First, a quick refresher on how hair works. Each strand grows from a tiny pocket called a follicle, and that follicle spends its life cycling through four distinct phases: anagen (the active growth period), catagen (a short transition), telogen (a resting stage), and finally exogen, when the hair falls out. On average, the scalp adds about five inches of length each year, but the length of the anagen phase can range from two to eight years for any given follicle, and that window shrinks as we age.

Lots of things can tug at that cycle—genetics, diet, stress, hormones, even certain medicines. “Thyroid problems, iron deficiency, major illness or rapid weight loss can push a lot of hairs into the shedding phase,” explains board‑certified dermatologist Elizabeth Bahar Houshmand, who contributed to a recent Popular Science piece. In other words, the internal health of the follicle matters far more than the temperature outside.

So where does the summer story come from? Some point to the boost in vitamin D that sunshine provides, assuming more vitamin D means faster hair growth. Houshmand is quick to debunk that: “If someone is truly vitamin D deficient, correcting it can support overall health, but there’s no solid evidence that extra vitamin D speeds up hair growth in people who already have normal levels.” In short, sunlight’s nutritional side‑effect isn’t the hair‑growth miracle people hope for.

Researchers have actually tried to measure seasonal differences. Two small studies from the 1990s looked at men’s scalp and beard hair. One found that the highest proportion of scalp follicles were in the growth phase in March, with a dip by September. Beard follicles, however, peaked in July, showing about a 60 % increase compared with winter. The sample sizes were tiny—just 14 and 10 participants—so you can’t take those numbers as a rule for everyone.

A larger 2009 study involving 823 women (both with and without alopecia) painted a clearer picture: during summer, most follicles were in the telogen, or resting, phase. In other words, the season when we think hair should be booming is actually when it’s most likely to be taking a break.

Even if the beard does grow a bit faster in the heat, the math works out to roughly three‑tenths of an inch extra per month—hardly enough to notice without a ruler. The practical takeaway? Sun‑safe habits are great for mood and overall health, but moving to a sunnier climate won’t magically fill in a receding hairline.

Bottom line: your hair doesn’t care about the calendar as much as it cares about your internal health. Keep a balanced diet, manage stress, and give your scalp a gentle scalp‑care routine. That’s the real recipe for keeping the strands looking their best, no matter the season.