There’s a lot we know about hair, but some aspects still remain a mystery, including why straight hair suddenly becomes curly, or vice versa. Most hair doesn’t just change drastically, although it has been known to happen. Straight hair becoming curly suggests that something is happening to the shape of the hair follicles, but no one knows why that happens or what causes it. Hormones are believed to play a role, but there’s no hard evidence to confirm this.

Aging

Our hair changes naturally as we age and particularly when we enter midlife; it becomes finer, less full, and many people experience hair thinning or loss. All of that is inevitable, but no one expects their straight hair to go curly. It’s unknown how straight hair with a rounder follicle suddenly transforms into a flatter follicle, creating curly hair.

Chemotherapy

Cancer patients who’ve undergone chemotherapy often find that their formerly straight hair grows back in curly. This may be due to the fact that chemo has an effect on rapidly growing cells, including hair follicles. When jolted by the chemo, the follicle changes temporarily but eventually returns to its regular shape.

Muscular Changes in the Base of Hair Follicles

Changes in the muscles at the base of each hair follicle may be the reason hair textures sometimes change, according to Jonathan Torch, founder of Toronto’s Curly Hair Institute and creator of Curly Hair Solutions. Muscular changes frequently occur during puberty, menopause, and chemotherapy—medications and hormones alter the muscle tone of the hair follicle. Sometimes the change in hair texture can be extreme, like going from straight to curly, or vice versa.

Hair Follicles Become Altered Over Time

As you age, your hair loses pigment and turns gray, and your follicles may grow more slowly, too. When there is slow follicle growth, hair can fall out, and when it does grow back in it may be coarser than it was before.