Pupil size dynamics during prolonged wakefulness reflects the dual interaction of sleep-homeostasis and the circadian timing system and is related to cortical excitability

Objective. We sought to characterize daily variations in pupil size as a function of sleep need and circadian phase. We also assessed second-to-second pupil size variability during prolonged wakefulness.

Methods. Twenty-two healthy young men (22 y.o. ± 2.6) followed a 29h sleep deprivation protocol under constant routine conditions. On twelve occasions, pupil size was recorded (90 Hz sampling rate) while fixating a dot and suppressing eye blinks. Following automatic eye blink and artefact rejection, mean pupil size and average point to point variation in pupil size data were computed. Data were realigned according to individual dim-light melatonin onset determined based on hourly saliva samples.

Results. Preliminary analyses indicate that both mean pupil size and pupil size variability show a main effect of circadian phase (PROC MIXED; n = 20; F11,206 > 4.4, p < 0.001). Post hoc analyses show that mean pupil size and pupil size variability increase up to the evening wake maintenance prior to decreasing until the early morning around the putative sleep promoting zone.

Conclusion. These data confirm the pupil size and pupil size variability reflect the dual interaction of sleep homeostasis and the circadian timing system. Further analyses will determine how pupil size dynamics relates to makers of brain function.