Circadian rhythms in the human pupil and eyelid

The human eye was studied to investigate its use as a noninvasive marker of circadian phase state. Subjects were measured every 30 min over 24 h to observe bilateral changes in pupil size and palpebral fissure. Twelve males and twelve females (ages 18-29 yr, mean = 21.6 yr) were studied around the clock while they remained within a light-controlled facility (< 100 lux). The acrophases (maximums) for pupil diameters and palpebral fissure distances were estimated using a cosine curve-fitting technique applied to each 24-h data set. Pupil diameter acrophases occurred randomly throughout the day. The data demonstrated a significant circadian rhythm for both resting and maximum palpebral fissure values. However, palpebral fissure acrophases were widely distributed, with the largest portion of the peaks occurring between 10:00 A.M. and 10:00 P.M.. These data did not support the hypotheses that sympathetic innervation to the pineal is paralleled in the eye, and results do not demonstrate any applicability of human eye measurements as circadian markers.