Sleepiness and Fatigue as Consequences of Cumulative Sleep Restriction: Insights from Fine-Grained Subjective Measures and Skin Temperature in the Field.

Diagnosis and monitoring of daytime sleepiness remain challenging and are strongly reliant on subjective assessments. To revisit common monitoring tools and explore new assessment modalities, we investigated the response of daily and momentary subjective measures of sleepiness and fatigue and skin temperature to sleep restriction, assessed between- and within-day variations in these responses, and studied their convergence. Seventeen healthy participants (aged 19-32 years, seven females, ten males) participated in a field study employing ecological momentary assessment. After a one-week baseline, two sleep conditions (4 h/night vs. 7-9 h/night, actigraphy-controlled; three nights each) were counterbalanced across participants. During the experimental conditions, sleepiness and fatigue were assessed using subjective rating scales administered in daily diary questionnaires and experience sampling questionnaires (10 notifications per day), while distal and proximal skin temperatures were continuously recorded. Results revealed significant effects of sleep restriction on distal and proximal skin temperature and daily reports of sleepiness and fatigue, independent of the number of sleep-restricted nights. The effects on momentary assessments were moderated by day, reflecting a cumulative effect of the sleep restriction from Days 1 to 3. The effects of sleep restriction on momentary sleepiness and fatigue ratings or hourly skin temperature metrics were not significantly moderated by time of day. Hourly skin temperatures (distal skin temperature and temperature gradient) were significantly related to subjective sleepiness and fatigue. In conclusion, all self-reports were sensitive to the sleep restriction, but momentary assessments illustrate the manipulation's cumulative effects and captured temporal dynamics in sleepiness and fatigue within days. This investigation showed overlap between sleepiness and fatigue experiences as reflected in medium to strong associations. Skin temperature correlates with momentary subjective sleepiness (and fatigue); however, considering it a proxy for daytime sleepiness remains exploratory.