The researchers were able to analyze how the hair shaft changes over time. See, under an intense microscope, you can tell a lot about a hair’s journey: Each strand is made up of dead cells, but those cells are very much alive before they’re uprooted for the scalp. “While hairs are growing, cells receive chemical and electrical signals from inside the body, including stress hormones,” the report reads. When those signals occur, it can cause changes in the proteins and molecules inside the hair shaft—and those changes remain there even once the strand grows out of your head. Neat, no?

Each shift that happens inside the strand offers a little message: “Just like tree rings hold information about past decades, and rocks hold information about past centuries, hairs hold information about past months and years,” the report continues. That’s why the researchers wanted to measure these hair pigmentation patterns in tandem with times of stress and see if any of the changes matched up. 

Participants were asked to identify the most and least stressful events over the last 12 months, mark them on a map, and connect the dots to create a visual timeline of ups and downs. Then they measured each hair to match the specific weeks or months along the stress profile. It turns out, stressful moments (like marital conflicts, according to one participant) were associated with loss of hair pigment, whereas during moments of bliss the hair regained its pigment. 

“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,” says senior author Martin Picard, Ph.D., in a news release.