An N-of-1 Investigation into Stress-Related Hemodynamics in the Prefrontal Cortex During the First Sleep Cycle*

It is well-known that stress affects sleep quality, suggesting abnormal brain activity during sleep when people are stressed. However, no study has examined where brain processes stress during sleep. This study aims to explore the associations between bedtime stress and the hemodynamics in the prefrontal cortex (PFC) during the first sleep cycle under free-living conditions. Stress biomarkers including salivary cortisol and secretory immunoglobulin A (sIgA) were measured using the SOMA Dual Analyte LFD test kits on the experiment nights between 22:00-23:00 to control the circadian oscillation of the stress-related hormones. Perceived stress level was rated on a 1-10 Likert scale right after the collection of the salivary samples. The hemodynamics of the pre-frontal cortex (PFC) was measured using a wearable functional near-infrared spectroscopy (fNIRS) device. Correlation analysis with statistical test was performed to examine the associations between different stress indicators and a set of time-domain and frequency-domain features derived from the hemodynamic responses. Significantly positive linear correlations were observed in the standard deviation, skewness, and kurtosis between the average concentration change of oxyhemoglobin and that of deoxyhemoglobin in the whole region of interest. Stress was found to correlate to the hemodynamics in the mid-DLPFC, the caudal-DLPFC, and the left RLPFC. The relationships between stress and these PFC subregions depends on the stress indicator adopted. Our finding provides supplementary support to the role of the PFC in processing stress. The preliminary results also shed light on the development of stress response markers in brain activity that can be measured with wearable brain-computer interface technologies.