Skin conductance measurements reveal link between insomnia and disrupted fear regulation

A new computational model using skin conductance measurements to estimate stress-related emotional arousal found a significant difference in learned fear and safety responses over time between adults with and without insomnia. The new study from Dr. Rose T. Faghih of New York University Tandon School of Engineering, US, and collaborators at Harvard Medical School, US, describing the work was published this week in the open-access journal PLOS Mental Health.

Insomnia Disorder, affecting an estimated 10% of the population, has been widely conceptualized as a disorder of chronic hyperarousal linked to emotional and autonomic dysregulation. Sleep is thought to help consolidate fear extinction, the process of learning that a previously threatening stimulus no longer signals danger, forming a new, inhibitory "safety memory" to override an established "fear memory." 

Skin conductance, changes in the skin's electrical properties driven by sweat gland activity, is a well-established marker of stress and emotional arousal. Previous studies looking at skin conductance and fear extinction have focused primarily on sharp, isolated incidences of stress activity.

In the new study, researchers designed two statistical models estimating stress and arousal from skin conductance: one model using only isolated stress spikes, and one model that also incorporated the signal's continuous background levels. Both models were applied to skin conductance recordings from an initial cohort of 24 adults with insomnia and 24 good sleepers who completed a task in which they were temporarily conditioned to fear a visual stimulus, then learned it was no longer dangerous.

Only the model including the tonic background signal detected a significant difference between the two groups. From the conditioning phase, when the visual stimulus was paired with a mild shock, to the extinction phase, when it no longer was, the average 'high-arousal index' increased in people with insomnia but slightly decreased in good sleepers. Other differences between the groups - including lower average arousal during conditioning and higher average arousal when the fear and safety memories were recalled on the second day - followed a similar pattern but did not reach statistical significance.

These findings suggest that because people with insomnia failed to fully acquire the safety memory on the first day or retain it until the second day, the new extinction memory was too weak to oppose the original fear memory during recall.

Lead author Tianyu Hwang summarizes the experiment as follows: "Our study shows that insomnia isn't just about poor sleep-it directly relates to the body's ability to learn and remember that a threat is gone." 

"Hyperarousal is thought to be an underlying risk factor for developing insomnia, and we were excited to see that sophisticated mathematical parsing of skin conductance revealed the same effect we had previously observed using fMRI," co-author Dr Edward Pace-Schott writes. "This hyperarousal could oppose subsequent learning that a stimulus has become safe, which may explain why anxiety treatments like exposure therapy are more successful in people with good sleep."

Senior author Dr Rose Faghih adds: "Using our previously developed sympathetic arousal virtual sensors based on microfluctuations in sweat activity, we compared insomnia patients and healthy controls during fear extinction and observed that insomnia is associated with a reduced capacity to extinguish physiological threat responses." 

In our study, people with insomnia, compared to their good sleeping peers, became more aroused once the danger was taken away. For patients, this may help explain why the fear that 'a bad night will ruin tomorrow' survives so many tomorrows that turn out just fine."

Dr. Ryan Bottary, co-author

Co-author Dr Saman Khazaei concludes: "I find it interesting to see that decoded arousal from certain components of skin conductance reflects a signature of a disrupted fear response in people diagnosed with insomnia. This finding may motivate further research on skin conductance and the use of wearable devices that measure it to improve sleep in everyday-life settings."