I’ll highlight this 2021 human study’s findings regarding stress:

“We profiled hair pigmentation patterns (HPPs) along individual human hair shafts, producing quantifiable physical timescales of rapid greying transitions. White/grey hairs that naturally regain pigmentation across sex, ethnicities, ages, and body regions, quantitatively define reversibility of greying in humans.

A systematic survey of two-colored hairs on the scalp of a 35-year-old Caucasian male with auburn hair color over a 2-day period yielded five two-colored hair shafts (HSs) from the frontal and temporal scalp regions. Unexpectedly, all HSs exhibited reversal. HPP analysis further showed that all HSs underwent reversal of greying around the same time period.

A retrospective assessment of psychosocial stress levels using a time-anchored visual analog scale (participants rate and link specific life events with start and end dates) was then compared to HPPs. Reversal of greying for all hairs coincided closely with decline in stress and a 1-month period of lowest stress over the past year (0 on a scale of 0–10) following a 2-week vacation.

We were also able to examine a two-colored hair characterized by an unusual pattern of complete HS greying followed by rapid and complete reversal plucked from the scalp of a 30-year-old Asian female participant with black hair. HPP analysis of this HS showed a white segment representing approximately 2 cm.

Quantitative life stress assessment revealed a specific 2-month period associated with an objective life stressor (marital conflict and separation, concluded with relocation) where the participant rated her perceived stress as highest (9–10 out of 10) over the past year. The increase in stress corresponded in time with complete but reversible hair greying.

We document a complete switch-on/off phenomena during a single anagen cycle. Proteomic features of hair greying directly implicate multiple metabolic pathways that are both reversible in nature and sensitive to stress-related neuroendocrine factors.

This new method to quantitatively map recent life history in HPPs provides an opportunity to longitudinally examine the influence of recent life exposures on human biology. Additional prospective studies with larger sample sizes are needed to confirm robust reproducibility and generalizability of our findings.”

https://elifesciences.org/articles/67437 “Quantitative mapping of human hair greying and reversal in relation to life stress”