Feeling sleepy? It's possible to tell from your eyes, according to a small new study.
Looking at how a person's pupils are able to respond to stimuli -- how fast they respond, and to what degree -- in addition to looking at his or her blink rate, is a reliable way to measure how alert he or she is, according to the new study published in the International Journal of Bioinformatics Research and Applications. Their study was conducted on 35 men.
Previously, alertness was mainly dictated by pupil dilation and contraction. But the researchers, from the Tokyo Institute of Technology and Aichi Medical University, noted that also taking into account the blinking rate is able to better indicate how sleepy a person is.
The findings could be applied to test the sleepiness of drivers or workers in the truck-driving or machinery-operation fields, the researchers said.
So what about that heavy feeling you get in your eyelids before you finally doze off? That's a matter of muscle fatigue, kind of like how "your legs would become tired from walking for a whole day," physiology professor Mark A. W. Andrews, of Seton Hill University, told NBC News.
For more signs you may be more tired than you realize, click through the slideshow: