SAD Lamp Math: Why 10,000 Lux Means Nothing Without the Inches

The clocks fell back yesterday, and if your evenings already feel shorter, you’re not imagining it. This is the week a lot of us finally order a SAD lamp, and almost every box on the shelf promises the same thing: 10,000 lux. But when researchers put 24 light therapy devices on a light meter in 2019, only 7 met their clinical criteria, and every one of those 7 was a larger light box. Some of the “10,000 lux” products only hit that number when you sat unrealistically close to them. The number on the box is real. It just isn’t measured where your face will be.

So before you spend $43 or $131 or $300, ask the one question that turns the lux claim into information: at how many inches?

The 10,000 lux on your SAD lamp box is measured at a distance you won’t sit at

Lux is a measure of how much light lands on a surface, which means it depends entirely on how far that surface is from the source. Move a small lamp twice as far from your eyes and you can lose a big chunk of the light. A large panel loses brightness more slowly as you back away, which is a big part of why big boxes kept passing in the lab and small ones kept failing.

The 2019 study, by Mark Oldham, Mary Oldham, and Paul Desan in Psychiatric Research and Clinical Practice, set a fairly modest bar. A device had to deliver at least 7,000 lux at its own recommended distance (or at 12 inches, if the maker recommended sitting closer), still give at least 5,000 lux when you shifted 6 inches off center, and do it without harsh glare. That middle test matters more than it sounds. Nobody sits perfectly still for half an hour while drinking coffee and scrolling email. One small device in the study measured just 4,600 lux at its recommended 12 inches. Others did fine straight on but lost the dose the moment you leaned.

Even the experts don’t agree on where to sit, which tells you how much the box claim depends on the box. Yale’s winter depression program uses about 11 inches as its testing distance. The Carex Day-Light Classic Plus, one of the big boxes clinicians often mention, is rated for 10,000 lux at 12 to 14 inches. Verilux says most of its HappyLight models deliver 10,000 lux within “no more than 24 inches.” The Cleveland Clinic tells patients to sit two to three feet away. Those aren’t small differences. A lamp that’s perfect at 12 inches can be a nightlight at 30.

Why a dimmer lamp costs you time instead of money

Light therapy works on a dose, which is brightness multiplied by time. Yale treats 30 minutes at 10,000 lux as roughly equal to 60 minutes at 5,000 lux. That’s the trade the manufacturers don’t put on the front of the box: a dimmer lamp doesn’t necessarily fail, it just asks for more of your morning.

Let me run the math with real prices. The Verilux HappyLight Lumi Plus lists at $42.99 on Verilux’s site. The Carex Day-Light Classic Plus is $131.04 at Walmart. That’s an $88.05 gap. Now suppose your compact lamp, at the 12 inches you’ll actually sit, reads 4,600 lux on a phone light meter (the same reading the dimmest device in the 2019 test gave at its recommended distance). I’m not saying the Lumi Plus measures that. I’m saying that’s what to check for. To get the same dose as 30 minutes at 10,000 lux, you’d need 30 times 10,000 divided by 4,600, or about 65 minutes each morning. That’s an extra 35 minutes a day.

From today, November 2, through March 31, there are 150 mornings. One hundred fifty times 35.2 extra minutes comes to roughly 88 hours. So the cheaper lamp saves you $88.05 and costs you about 88 hours of sitting still in front of a light, which works out to about $1 an hour for your time. Most of us would never take that deal if someone offered it out loud. And the realistic outcome is worse than the math, because the session that runs past an hour is the one you quietly stop doing by mid-December.

The flip side: if your phone meter shows the cheap lamp reading 10,000 lux at a distance you can comfortably keep, it’s a perfectly good lamp. Price isn’t the signal. The reading is.

A two-minute phone test tells you whether to keep it

The 2019 paper and Yale both point out that free phone apps that read lux can give you a rough check at home. Phone sensors aren’t lab equipment, so treat the result as a ballpark, not a certificate. Set the lamp up where you’ll actually use it, at your kitchen table or desk, at a normal sitting distance. Hold your phone at eye level with the screen side facing the lamp, since most apps read from the front sensor. Note the number. Then lean 6 inches to one side, the way you would reaching for your mug, and check again.

If the straight-on reading is near 10,000, you can follow the standard 20 to 30 minutes. If it’s around 7,000, budget closer to 43 minutes. If it drops below 5,000 when you lean, that lamp is going to keep shortchanging you every time you move. That’s when it’s worth returning it while you still can and putting the money toward a larger panel. Yale’s list of devices that meet its criteria includes larger units like the NorthStar 10,000 and SunRay II, plus smaller ones such as the Day-Light Sky and BOXelite that can work if you position them carefully.

Timing matters as much as brightness, and it starts now

The other half of the dose is when you take it. Yale’s protocol calls for 30 minutes at 10,000 lux before 8 a.m., every day, and the Cleveland Clinic also says morning sessions seem to work best. Michael Terman of the Center for Environmental Therapeutics told NPR in 2025 that fall is the time to start, before the darkest months arrive, and that a larger screen angled down toward your eyes cuts glare and keeps the dose steady when your head moves. Some people notice a difference within days, though the Cleveland Clinic says full benefit can take about two weeks.

For context, about 5% of U.S. adults have seasonal affective disorder, according to the Cleveland Clinic, and another 10% to 20% get a milder case of winter blues. It’s more common in women and usually starts between ages 18 and 30. If what you’re feeling is heavier than a slump (if it’s affecting your sleep, your appetite, your work, or how you feel about yourself), a lamp is not a substitute for talking to someone. We’ve written about when it’s time to seek professional help, and that conversation is worth having before you self-treat.

Talk to your doctor before starting light therapy, too, especially if you have bipolar disorder, diabetes, or a retinal condition, or take medications that make you sensitive to light. The Cleveland Clinic flags all of these. Bright light can trigger hypomania in people with bipolar disorder, and some antibiotics and anti-inflammatories raise light sensitivity.

If you’re fine to go ahead, my advice is simple: buy the SAD lamp, then test it this week while the return window is open. Ten thousand lux at 12 inches is a treatment. Ten thousand lux at a distance nobody sits at is marketing. For more ways to get through the darker months, our guide to coping with anxiety and depression has a broader toolkit.