You've probably never noticed your office lights flickering. Most people don't. But here's an unsettling fact: some of the most common light sources around us are turning on and off more than 100 times per second. And even though you can't see it happening, your body knows.
That persistent headache you get at work? The eye strain after a few hours at your desk? The fatigue that seems to come from nowhere? Before you blame stress or too much screen time, you might want to look up at your lights.
The Flicker You Can't See
Light flicker is the rapid variation in light output over time. It happens when light sources are powered by alternating current (AC) without proper smoothing. The electricity from your wall socket alternates at 50 or 60 Hz, which means the power feeding most lights is cycling on and off 100 or 120 times per second.
Traditional incandescent bulbs handle this gracefully. Their filaments heat up and stay warm between power cycles, creating essentially smooth, continuous light. The flicker is minimal and rarely causes problems.
But LEDs don't have thermal inertia. They respond instantly to changes in power. And when paired with cheap drivers—the electronic components that convert AC to the DC power LEDs need—they can flicker dramatically at those same 100-120 Hz frequencies.
Here's the strange part: this flicker is technically "invisible." Most humans can't consciously perceive flicker above 60-90 Hz. It doesn't look like the lights are strobing. Everything appears perfectly steady.
Yet the biological effects are very real.
What the Science Shows
The evidence that "invisible" flicker affects human health has been building for decades. Even when flicker occurs above the critical fusion frequency where we can't consciously see it, measurable biological effects persist.
Electroretinogram (ERG) studies have demonstrated that invisible flicker is transmitted through the retina. Your eyes are registering these rapid changes even when your conscious mind isn't aware of them. The signal reaches your brain, triggering physiological responses.
One of the most striking studies comes from Brundrett in 1974, who surveyed 627 office workers in the UK working under magnetic-ballast fluorescent lights. These old-style fluorescents are notorious flicker sources. The results were stark:
45% reported headaches
40% reported eyestrain
And here's the crucial detail: those reporting headaches were significantly more likely to also report seeing flicker, even though the flicker frequency was supposedly invisible.
A small subset of people—about 8% in the survey—averaged more than 2 headache days per week under flickering lighting. For them, the impact wasn't minor discomfort. It was a chronic health issue directly linked to their environment.
The Double-Blind Evidence
Perhaps the most compelling proof comes from a double-blind office study. Researchers replaced highly flickering magnetic ballast fluorescent lights with low-flicker electronic ballast fluorescents—but they didn't tell the workers.
The workers had no idea when the lights had been changed. There was no placebo effect, no expectation bias. Yet the rates of headaches and eye strain changed markedly between the two lighting conditions.
This confirms what many have suspected: flicker is a real biological stressor, not an imagined one. Your body responds to it whether you're consciously aware of it or not.
Who's Most Affected?
More recent research has focused on particularly vulnerable populations. A 2024 study by Miller and colleagues examined people with migraine compared to healthy controls. Participants observed flicker patterns commonly found in architectural lighting for 60-120 minutes.
The results were striking:
41% of people with migraine developed headaches during or after the session, compared to just 8% of controls. Even more concerning, 64% of people with a history of migraine experienced discomfort or an adverse reaction within 4 hours of the experiment, compared to 19% of control individuals.
For people prone to migraines, flicker isn't just uncomfortable—it's a direct trigger.
Other documented symptoms of flicker exposure include:
- Fatigue and reduced productivity
- Blurred vision
- Dizziness and nausea
- Increased heart rate
- In rare cases, photo-induced seizures in susceptible individuals
The LED Problem
When LEDs first entered the consumer market, they were praised for their energy efficiency. And to be fair, they do use far less electricity than incandescent bulbs. But the early rush to produce cheap LED bulbs created a quality crisis.
The driver—the small circuit board inside every LED bulb that converts AC power to DC—is expensive to do well. Manufacturers looking to compete on price often cut corners here, using simple rectifiers that don't adequately smooth the power supply.
The result is LEDs that flicker at 100-120 Hz, exactly like old fluorescent lights.
Not all LEDs are created equal. High-quality LED bulbs use proper DC drivers or high-frequency switching that eliminates perceptible flicker. The modulation depth can be reduced to less than 8%, making them essentially flicker-free. These bulbs exist. But they cost more, and they're not always easy to identify on the shelf.
The cheaper the LED bulb, the more likely it is to have significant flicker. That bargain multi-pack from the hardware store? There's a good chance it's flickering heavily.
Testing for Flicker
The easiest way to check for flicker is with your smartphone. Open your camera app and switch to slow-motion mode if available. Point it at the light source. If you see rolling bands or pulsing in the video feed, you're looking at flicker.
You can also try the "moving object test." Wave a pen or your hand quickly in front of the light. If you see distinct, separate images rather than a smooth blur, that's the stroboscopic effect of flicker in action.
Professional lighting designers use dedicated flicker meters that measure percent flicker and flicker index. The IEEE developed a standard (PAR1789) that recommends specific limits based on frequency. Quality lighting should have less than 10% flicker at any frequency, with less than 3% being ideal.
What You Can Do
If you suspect flicker is affecting you, here are practical steps:
1. Identify your light sources. Focus on the spaces where you spend the most time. Offices, kitchens, and workspaces are priorities.
2. Test for flicker using the smartphone camera method. You might be surprised what you find.
3. Replace high-flicker sources. Look for LED bulbs specifically marketed as "flicker-free" or with specifications showing low percent flicker. Yes, they cost more. The health benefits are worth it.
4. Consider incandescent or halogen bulbs for critical spaces. They remain available for specialty uses and provide inherently flicker-free light. An incandescent desk lamp might be the simplest solution for your workspace.
5. If you're in an office with fluorescent lights, check if they use magnetic ballasts. These are usually in older buildings. Electronic ballasts operate at much higher frequencies (20,000-60,000 Hz) and effectively eliminate the problem. It might be worth requesting an upgrade.
6. For sensitive individuals, invest in quality. If you're among the 8% who get frequent headaches from flicker, this isn't a luxury—it's a necessity. Seek out high-CRI, flicker-free LED bulbs from reputable manufacturers.
The Bigger Picture
The flicker issue highlights a larger problem with our transition to new lighting technologies. In the rush to save energy, we've sometimes sacrificed the quality factors that matter for human health.
Flicker is invisible until it isn't. Most people can't point to their lights and say "those are making me sick." The symptoms are diffuse—headaches, fatigue, eye strain—easily attributed to other causes. But the controlled studies are clear: for a significant portion of the population, flicker has real, measurable health impacts.
The good news is that we don't have to choose between energy efficiency and health. High-quality LEDs exist that offer both. Better awareness, better standards, and better consumer information can drive the market toward bulbs that are good for both the planet and the people living on it.
Your lights should be invisible in the best way—not because they're flickering above your conscious threshold, but because they're so comfortable you never think about them at all. Until you do, and realize how much better you feel when the flicker stops.