Pupillometry

Pupil size decreases in the evening, a phenomenon called evening meiosis, related to the same parasympathetic shift that causes heavy eyelids.

Pupillometry, measuring pupil size and fluctuations, is used by researchers as an objective measure of drowsiness.

Electrooculography, which measures the electrical potential between the front and back of the eye, can detect slow eyelid closures.

Researchers have found that the average number of slow closures per minute increases from near zero in the morning to five or more in the hour before rest.