“just as the rings in a tree trunk hold information about past decades in the life of a tree, our hair contains information about our biological history,” picard said. “when hairs are still under the skin as follicles, they are subject to the influence of stress hormones and other things happening in our mind and body. once hairs grow out of the scalp, they harden and permanently crystallize these exposures into a stable form.”
using a new method to capture high resolution images — developed by the study’s first author, ayelet rosenberg — researchers were able to examine slices of hair just 1/20
th
of a millimetre in size that would have taken an hour to grow on a human head.
“if you use your eyes to look at a hair, it will seem like it’s the same colour throughout unless there is a major transition,” picard said. “under a high-resolution scanner, you see small, subtle variations in colour, and that’s what we’re measuring.”
by comparing the minute hair fragments of 14 volunteers to a stress diary they were required to keep, researchers found ample evidence connecting stress to the greying of hair and, in some cases, to the hair returning to its natural colour — a process that has not been quantitatively documented in the past. “there was one individual who went on vacation and five hairs on that person’s head reverted back to dark during the vacation, synchronized in time,” picard said.