Thirty minutes at 10,000 lux. It's the number that appears on every SAD lamp box, every light therapy guideline, every NHS self-help page about seasonal depression. It's presented as though it were a precise medical prescription.
It isn't. It's a clinical convention that emerged from trials conducted with the equipment available in the 1980s, and it's been repeated so many times that most people assume there's more precision behind it than there actually is.
Where the 30-Minute Rule Came From
The early SAD light therapy trials used 2,500-lux broad white light sources and found that two hours of exposure produced significant effects. Later trials with 10,000-lux sources found that 30 minutes produced similar results, and the 30-minute/10,000-lux combination became the standard clinical recommendation.
It works. But it was derived empirically — by testing what amount of available technology produced sufficient effect — rather than from a fundamental understanding of what the biology actually requires. For the evidence on how light therapy performs clinically, including its comparison to antidepressants, see our light therapy vs antidepressants guide.
What the Biology Actually Requires
Your circadian clock responds to a melanopsin-weighted photon dose: the total number of photons in the 480–490nm range that reach your retinal ganglion cells over a given period. The 10,000-lux/30-minute recommendation delivers this dose using broad white light, of which perhaps 5–10% is in the biologically active range.
What varies between individuals: the density of melanopsin-expressing cells in the retina. Research suggests this varies by a factor of roughly 3–4x between individuals, meaning the same lamp at the same distance for the same duration produces significantly different biological effects for different people.
What also varies: age. The crystalline lens of the eye yellows with age, absorbing blue-cyan wavelengths before they reach the retina. By age 70, the lens transmits roughly 50% of 480nm light compared to a young adult. The same lamp for the same duration delivers half the melanopsin dose to an older person.
A Smarter Way to Think About It
Rather than asking "how long should I use it?", the right question is "have I received enough melanopsin stimulus today?" This requires knowing your actual light intake — your melanopsin-weighted dose for the day — relative to a target that accounts for your latitude, the season, and your individual response.
This is the light health score at the core of the LightHealth system. Instead of a fixed protocol, it gives you real-time feedback on whether your circadian clock has received the signal it needs — and adjusts the recommendation based on the light you've already received from natural sources that day.
On a bright summer morning when you've spent time outdoors, you may need no supplementary light at all. On a dark January morning in Edinburgh, you may need more than the standard 30 minutes of a conventional SAD lamp to hit the melanopsin threshold. For a guide to the devices currently available, see our best SAD lamp 2026 guide.
Learn about the LightHealth dose tracking approach.
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