When a police officer shines a flashlight in your eyes, they’re almost always checking for signs of drug or alcohol impairment. Your pupils, eye movements, and the blood vessels on the surface of your eyes all change in specific, predictable ways depending on what substances are in your system. That quick beam of light gives an officer a surprising amount of information in just a few seconds.
Checking Your Pupil Size and Reaction
Your pupils naturally constrict when light hits them and dilate in darkness. Drugs interfere with this reflex in distinct ways, and officers are trained to recognize the patterns. Stimulants like cocaine and methamphetamine cause dilated pupils that react slowly to light. Opiates like heroin do the opposite, constricting pupils to pinpoints with little to no response when light is introduced. Hallucinogens such as LSD and psilocybin dilate the pupils but, unlike stimulants, the pupils still react to light at a normal speed.
These differences matter because they help officers narrow down the category of substance involved. An officer who sees your pupils are tiny and barely responding to a flashlight beam has a very different suspicion than one who sees wide, sluggish pupils. Cannabis is trickier: studies have found it can cause either constriction or dilation depending on the individual, but it often produces a noticeably decreased pupillary light reflex, meaning the pupils are slower to contract when exposed to light.
Before even testing each eye’s light response, officers are trained to check whether both pupils are the same size. Unequal pupils can indicate certain medical conditions rather than impairment, and noticing this early helps prevent a misinterpretation of the results.
The Horizontal Gaze Nystagmus Test
The flashlight often doubles as the stimulus for the horizontal gaze nystagmus test, one of three standardized field sobriety tests endorsed by the National Highway Traffic Safety Administration. In this test, the officer holds a penlight or small flashlight about 12 to 15 inches from your face and slowly moves it side to side while asking you to follow it with your eyes only, keeping your head still.
They’re watching for three specific things in each eye, for a total of six possible indicators. First, whether your eyes can track the moving light smoothly. Healthy, sober eyes follow a moving target in one fluid motion. Alcohol and sedating drugs disrupt this ability, causing the eyes to jerk or stutter as they try to keep up. This is called “lack of smooth pursuit.”
Second, the officer looks for involuntary jerking of the eye when it’s held at the far left or far right position (maximum deviation). If this nystagmus is distinct and sustained for at least four seconds, it counts as a clue.
Third, and most telling, is whether the jerking begins before the eye reaches a 45-degree angle from center. In sober individuals, nystagmus typically only appears at extreme angles. When it starts earlier, it strongly correlates with a high blood alcohol level.
Any sedating substance can disrupt smooth pursuit, including prescription medications like anticonvulsants and certain depressants. This is one reason officers also ask about medications during a stop.
Looking for Red Eyes and Other Visible Signs
The flashlight also lets officers get a clear look at the surface of your eyes. Cannabis, for instance, causes acute redness (conjunctival injection) that is one of the most reliable visible indicators of recent use. One study found that 94% of drug-impaired drivers who tested positive for THC in their blood had red eyes. Officers are also trained to look for eyelid tremors, which appeared in 86% of THC-positive subjects in one study, and droopy eyelids (ptosis), found in nearly 86% of blood THC-positive drivers during field sobriety evaluations.
Watery or glassy eyes, constricted blood vessels, or unusual coloring can all point toward different substances or medical issues. Without a focused light source, many of these signs would be invisible during a nighttime traffic stop.
Scanning the Inside of Your Vehicle
Not every flashlight aimed your direction is pointed at your eyes. Officers routinely use their flashlight to illuminate the interior of your car during a stop, particularly at night. Under the plain view doctrine, anything an officer can see from outside the vehicle without conducting a search is fair game. Court cases have established that using a flashlight to see what would already be visible in daylight doesn’t constitute a search.
In practice, this means officers are scanning for open alcohol containers, drug paraphernalia, loose plant material on surfaces, or weapons. In one court case, an officer spotted an open bottle of liquor, marijuana flakes on the center console, and the handle of a handgun between the seat and console, all with a flashlight beam through the window.
Disorientation as a Tactical Tool
There’s also a simpler, more tactical reason: a bright light pointed at someone’s face at night makes it very difficult for that person to see the officer behind it. High-output patrol flashlights can temporarily impair night vision, which gives an officer a safety advantage during uncertain encounters. The person being illuminated can’t easily assess the officer’s position, movements, or how many officers are present. This discourages aggressive behavior without requiring any physical contact.
The temporary blindness from a bright light in dark conditions fades within minutes, but those minutes are often all an officer needs to assess a situation and establish control of the interaction.
What Can Affect Your Results
Pupil responses and eye-tracking ability aren’t exclusively controlled by drugs and alcohol. Certain neurological conditions, brain injuries, and normal aging can all produce nystagmus or abnormal pupil reactions. Some people naturally have pupils of slightly different sizes, a harmless condition called anisocoria that affects up to 20% of the population. Fatigue, recent eye surgery, and even some over-the-counter medications can alter how your eyes respond to light.
The NHTSA guidelines instruct officers to position you away from flashing or strobe lights (like those on the patrol car) that could interfere with the test. Officers are also trained to note any medical explanations you offer. Still, the horizontal gaze nystagmus test is considered the most reliable of the three standard field sobriety tests, which is why it’s typically administered first. If you have a medical condition that affects your eyes, mentioning it during the stop creates a record that could matter later.