A new study to be published in Psychoneuroendocrinology links severe internet addiction with lower long-term cortisol levels measured in hair, suggesting a blunted physiological stress response among those most affected. The research team was led by Hideki Tsumura of Tokushima University and Shuhei Izawa of the National Institute of Occupational Safety and Health in Japan, who analysed hair samples from 155 university students.
The researchers reported that higher internet addiction symptom scores were associated with lower hair cortisol concentrations overall, and that individuals classified as highly addicted showed lower cortisol on average than those at lower risk. The findings point to a potential biological pattern in which chronic internet overuse coincides with an exhausted stress system.
Internet addiction is described in the study as a behavioural disorder characterised by excessive and poorly controlled online activity, spanning endless social media use, online gaming and shopping. It can cause distress or impair daily functioning, such as academic or work performance and social relationships. Psychologists have long linked internet addiction to psychological stress, with some models suggesting stress vulnerability drives people to seek online relief, though physiological data have yielded mixed results.
The research also examined testosterone, a hormone connected with reward processing and risk-taking, to see whether it might relate to internet addiction symptoms. While testosterone is produced by a different biological network and is thought to influence reward sensitivity, the study found no association between internet addiction scores and long-term hair testosterone concentrations. The lack of a link held across both male and female participants.
The study recruited 155 Japanese university students, comprising 94 women and 61 men with an average age of about 21. Researchers applied strict criteria to ensure viable biological samples, excluding individuals with endocrine disorders, acute psychiatric conditions, or those taking medications known to affect hormone levels, including hormonal contraceptives.
Participants completed the Internet Addiction Test, which assesses how often internet use disrupts sleep, academic performance or social interactions. They also completed another assessment to measure current depressive symptoms. Hair samples were taken from the back of each participant’s head, washed to remove contaminants, flash-frozen, and ground into a fine powder for hormone extraction and measurement.
In analysing the data, the researchers used statistical models that controlled for age, sex, body mass index and depression scores, and also accounted for hair care habits such as bleaching, perming and washing frequency.
The results showed a clear negative association between hair cortisol concentrations and internet addiction symptoms: those with more severe symptoms had lower levels of cortisol trapped in their hair. When split into groups by a standard internet addiction cutoff, the highly addicted group had lower cortisol than the low-risk group.
The study’s authors emphasised that the testosterone findings did not support their initial hypothesis and that results were consistent across sexes. They also noted that reduced hair cortisol has been observed in other conditions characterised by chronic stress or adversity, such as post-traumatic stress, severe depression and prolonged hardship, suggesting a shared biological pattern of an exhausted stress response in those with severe internet addiction.
Several limitations were acknowledged. The sample consisted entirely of young university students, which may not reflect the broader population or older adults in clinical settings. The study was cross-sectional, capturing a single moment in time, so it cannot determine whether a blunted cortisol response predisposes someone to internet addiction or results from it. The researchers also noted potential unmeasured factors, such as past trauma, that could influence the biological markers.
The study, titled “Association of internet addiction symptoms with hair cortisol and hair testosterone concentrations,” was authored by Hideki Tsumura and Shuhei Izawa. While the findings illuminate a possible biological signature of severe internet addiction, the authors caution that further longitudinal work is needed to unravel cause and effect.
