Pupil Size Can Objectively Identify Sleepiness, Sleep Deprivation

Pupillometry Often Used in Research Setting

The pupil size may correlate to the function of the nervous system and may vary with sleep deprivation and sleepiness
The pupil size may correlate to the function of the nervous system and may vary with sleep deprivation and sleepiness. Hans Solcer/Getty Images

It is often said that the eyes are the windows to the soul, but are the pupils the most obvious way to tell if someone is feeling sleepy? In a somewhat surprising phenomenon, measuring pupil size can be used to objectively identify sleepiness and the degree of sleep deprivation. It may correlate with the function of the central nervous system. How does this work? Learn about pupillometry and how this measurement may be used to identify how sleep deprived a person is.

What Is Pupillometry and Why Does Pupil Size Matter?

The measurement called pupillometry is most often used in research and consists of measuring the size of the pupil, which is the central black part of the eye. Not only the size, but how the pupil changes, can be meaningful.

Pupil size is determined by input from the nervous system acting on the muscles affecting the colored part of the eye called the iris. During periods of rest, or conversely during periods of activity and arousal, these influences will change. For example, if you are active, your sympathetic nervous system will take over and your pupils will dilate, allowing more information to be taken in. If you are about to be eaten by a lion, this system allows you to better spot it before it leaps on you.

The complementary parasympathetic nervous system will take over during periods of rest and relaxation, causing pupils to return to their default state and become smaller.

How Does Pupil Size Relate to Sleep Loss?

Research has shown that there is a strong relationship between sleep deprivation, pupil size, and pupil stability. A well-rested individual can maintain constant pupil size in darkness for 15 minutes. As you become more sleep deprived, your pupil size will become less stable.

It fluctuates (or oscillates) becoming subtly bigger and smaller rather than maintaining its size.

Moreover, your pupils overall size will shrink, perhaps reflecting fatigue in the task of maintaining the larger size. The muscles themselves may tire and the ability to keep the pupil open may fade.

Therefore, both pupil size and stability can objectively identify sleepiness and sleep deprivation. This might be useful to ensure adequate rest is attained and that the consequences of sleep loss do not occur. 

A Word from Verywell

Pupillometry is not widely used as it is mostly a research tool with the equipment not available much beyond this setting. You might imagine how it could be used, however. What if you could look into the camera of your smart phone and an app could make measurements and analyze your degree of sleepiness? This might be helpful to ensure that there is no impairment in driving or other activities that require a high level of attentiveness. Though measuring pupils is not routinely done currently, it may be a useful tool to develop in the future. Getting enough hours of sleep to feel fully rested may optimize the function of the body, including the pupils within the eyes.


Lowenstein, O et al. "Pupillary movements during acute and chronic fatigue: a new test for the objective evaluation of tiredness." Invest Opthalmo 1963;2:138.

Wilhelm, B et al. "Pupillographic assessment of sleepiness in sleep-deprived healthy subjects." Sleep 1998;21:258.

Yoss, R et al. "Pupil size and spontaneous pupillary waves associated with alertness, drowsiness and sleep." Neurology 1970;20:545.

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