Children who manage a long-lasting health condition carry more than medication schedules and clinic visits. Their bodies also hold a record of stress that can linger for months at a time. Scientists can now read part of that record in hair.

Emma Littler, a PhD candidate at the University of Waterloo, is the lead author of a new study that explains what those stress traces might mean.

Hair holds a record of stress

Hair is rich in clues because cortisol, a stress hormone, gets incorporated into each growing strand over time. The research shows that hair cortisol tracks long-term exposure rather than brief spikes.

This makes hair a useful biomarker for persistent load on the body. It is also easy to collect without needles or swabs.

Hair grows at roughly 0.4 inches each month, so a 1.2 inch segment reflects about three months. That time window is described in a previous study.

The measure used in research settings is called hair cortisol concentration (HCC). It offers an average rather than a snapshot, which matters for conditions that weigh on families for months and years.

Cortisol comes from the hypothalamic pituitary adrenal axis (HPA axis), the body’s core stress response system. When challenges pile up, this system can stay highly active or become blunted, and either pattern can relate to mood and behavior.

Tracking stress patterns

The new study followed 244 children in Canada who live with a chronic physical illness (CPI). The team tracked HCC over four years and compared those patterns to mental health symptom scores.

The researchers looked at the 1.2-inch segment nearest the scalp at each visit, which reflects recent months. They used modeling to group children by how their HCC changed over time.

Three clear HCC trajectories emerged. One group showed consistently high levels, one stayed low, and one started high but declined to lower levels.

Chronic illness and hair cortisol

Children who maintained high levels of HCC were more likely to report symptoms of psychopathology, including anxiety, mood, or behavior concerns. Individuals whose HCC declined over time tended to report fewer symptoms across follow-up.

More than two-thirds of the sample belonged to the high group. That is a strong hint that chronic illness can keep the stress system switched on for many kids.

“This study provides evidence that children with a CPI follow distinct HCC trajectories,” stated Littler.

Hair may help manage stress

If hair reveals sustained stress, then a child’s HCC might help clinicians flag who needs extra support sooner. It may also offer a way to check whether a support plan is easing stress over time.

Collection is simple, and it takes only a small lock from the back of the head. That ease matters for children who already have frequent medical appointments.

Hair cortisol concentration is not a diagnosis tool. It is a signal that can add context to what a child and family report.

What hair cortisol cannot tell us

An elevated HCC does not tell you which symptom a child might face. It cannot, on its own, explain why one child struggles while another adapts.

Patterns also vary by age and other factors. The research found that some children show a fall in hair cortisol concentration over time, and those children tended to report lower symptoms.

There are limits to any single marker. Many influences shape mental health, including family supports, school demands, sleep, pain, and treatment side effects.

Future research directions

Scientists are testing whether other biological signals can sharpen risk detection. A separate paper from the same research program reports that a small set of inflammatory markers in blood predicted changes in symptoms during the same four year window.

Those blood signals did not prove the exact cause, but they suggest that routine panels alongside mental health check ins could help doctors act earlier. Combining a noninvasive hair measure with periodic blood work and simple screening could form a practical care path.

Future work will also test how stress reducing supports influence HCC. If a child’s HCC drops as coping improves, that feedback could help families and care teams stay the course.

Sample diversity and access matter too. Building cohorts that reflect the many backgrounds and conditions children live with will make the science more useful for everyone.

The study is published in the journal Stress and Health.

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