The 10,000 lux figure on the side of a SAD lamp comes from a 1990 study in New York, which found that 30 minutes at 10,000 lux did about as much as the two hours at 2,500 lux used in the first light therapy trials of the 1980s. It was a convenience finding as much as a scientific one: people will sit in front of a lamp for half an hour before work, but not for two hours. This is the short history of how a research protocol became the number on the box.
Where did the idea of treating winter with light come from?
The modern story starts in the early 1980s at the National Institute of Mental Health in the United States. Norman Rosenthal and colleagues described a group of 29 patients whose depression arrived each autumn and lifted each spring, gave the pattern a name, seasonal affective disorder, and reported preliminary results from treating 11 of them by extending the winter day with bright artificial light. The 1984 paper in the Archives of General Psychiatry is the one everyone still cites.
Those first sessions were long and comparatively dim by today's standards: bright white fluorescent light of about 2,500 lux for three hours at dawn and three hours at dusk, for two weeks, compared with a dim yellow light of about 100 lux. The theory at the time was about lengthening the photoperiod, the number of daylight hours the body sees, rather than about a dose of light at a particular time.
How did 2,500 lux for two hours become the standard?
Through the rest of the 1980s, trials at several centres settled on a simpler routine: 2,500 lux for around two hours a day. In 1989 Michael Terman and colleagues pooled the data from 14 research centres, 332 patients in all, and found that morning sessions produced more remissions (53%) than evening (38%) or midday (32%) sessions. Timing had started to matter as much as duration.
Two hours every morning is a lot to ask of anyone with a job, a commute or children, and researchers knew it. The obvious question was whether a brighter light for a shorter time would do the same job.
The 1990 study that produced the number
In 1990, Terman's group published a trial in the Psychopharmacology Bulletin with the title "Efficacy of brief, intense light exposure for treatment of winter depression". Patients used 30-minute sessions at either 3,000 or 10,000 lux in the early morning, and 10,000 lux was also compared morning against evening. The remission rate was 75% with morning light at 10,000 lux, 25% with evening light at the same intensity and 19% at 3,000 lux. The authors concluded that 10,000 lux for 30 minutes was roughly equivalent to 2,500 lux for two hours, "suggesting reciprocity between intensity and duration", and reported no eye changes over two to six weeks of use.
| Period | Typical protocol | Source |
|---|---|---|
| 1984 | About 2,500 lux, three hours at dawn and three at dusk | Rosenthal et al., Archives of General Psychiatry |
| Late 1980s | 2,500 lux for about two hours, morning preferred | Terman et al. 1989 review of 14 centres |
| 1990 onwards | 10,000 lux for 30 minutes, soon after waking | Terman et al. 1990, Psychopharmacology Bulletin |
That is the whole origin of the figure. It was never a measurement of sunlight, and it is not a threshold below which nothing happens. It is the intensity at which a half-hour session matched the older two-hour one in a trial of a few dozen people.
Why does the distance matter so much?
Lux describes light arriving at a surface, and it falls away quickly with distance from a small source. A lamp that delivers 10,000 lux at 30 cm delivers far less at 60 cm. That is why the 1990 protocol specifies a distance as well as an intensity, and why a lamp specification without a distance is not a specification. Our guide to what 10,000 lux actually means goes through the arithmetic, and how far to sit from a light therapy lamp turns it into a habit. DAYARC Rise is rated at 10,000 lux at approximately 30–36 cm and DAYARC Mini at approximately 25 cm, and we print both distances because the number is meaningless without them.
What has changed since 1990?
Three things. Lamps moved from fluorescent tubes to LEDs, which is mostly a story about size, heat and running cost rather than effect; see LED vs fluorescent light therapy lamps. Researchers learned more about the eye's non-visual light receptors and why morning light shifts the body clock earlier, covered in how does a light therapy lamp work. And the evidence base grew, though not as much as anyone would like: a 2020 meta-analysis of 19 randomised trials found bright light modestly better than placebo for SAD, while the NHS still says there is not enough evidence to offer light therapy routinely. Do SAD lamps work? sets that out in full.
Frequently asked questions
Is 10,000 lux the same as sunlight?
No. A clear summer day is 32,000 to 100,000 lux in direct sun and 10,000 to 25,000 lux in the open shade; an overcast day is around 1,000 lux. The 10,000 lux figure was chosen because it worked in a 30-minute session, not because it copies the sun. How bright is a cloudy day in the UK? compares the numbers.
Does a brighter lamp work faster?
The 1990 study suggested intensity and duration trade off against each other, but only within the range tested. Very bright lamps are not a shortcut, and the NHS advises following the manufacturer's instructions and checking with a GP first if you have an eye condition or take medicines that increase light sensitivity.
Why 30 minutes rather than 20 or 45?
Because 30 minutes was the session length in the trial that established the figure. Many people use 20 to 30 minutes; some manufacturers and clinicians suggest up to 45 or 60 minutes at lower settings. How long should you use a light therapy lamp each day? covers the options.
Where does this leave you?
Knowing where the number comes from makes it easier to use well. The protocol behind it is 10,000 lux, at a stated distance, for about 30 minutes, soon after waking, most days. A lamp that quotes the lux without the distance, or a routine that skips the timing, is not the thing that was tested. If you want to try a routine, how to choose a SAD lamp in the UK lists what to look for, and the choose your light page compares the two DAYARC lamps.
About DAYARC
DAYARC is a new UK circadian rhythm startup. We make two bright-light lamps for the morning: DAYARC Rise (£199), a full-size lamp rated at 10,000 lux at approximately 30–36 cm, with four brightness settings, a 15 to 60-minute timer and adjustable height and angle; and DAYARC Mini (£49.99), a compact lamp rated at 10,000 lux at approximately 25 cm, with a folding stand that doubles as its cover. Both use UV-free LEDs and run from the mains. We publish plain specifications with the measuring distance, we do not make medical claims, and we write the DAYARC Journal to help people build a simple morning light routine around their own daily rhythm. You can read more about how we think on our approach page or compare the two lamps.
This article is general information, not medical advice. Check with a GP or qualified healthcare professional before using bright light if you have an eye condition, a condition affected by bright light such as bipolar disorder, or take medicines that increase your sensitivity to light. If you have persistent low mood or think you may have depression, speak to your GP. If you need urgent help, contact NHS 111, or call 999 in an emergency.
Sources
- Rosenthal N. E. et al. (1984). Seasonal affective disorder: a description of the syndrome and preliminary findings with light therapy. Archives of General Psychiatry, 41(1), 72–80
- Terman M. et al. (1989). Light therapy for seasonal affective disorder: a review of efficacy. Neuropsychopharmacology, 2(1), 1–22
- Terman J. S. et al. (1990). Efficacy of brief, intense light exposure for treatment of winter depression. Psychopharmacology Bulletin, 26(1), 3–11
- Campbell P. D., Miller A. M. and Woesner M. E. (2017). Bright Light Therapy: Seasonal Affective Disorder and Beyond. Einstein Journal of Biology and Medicine, 32, E13–E25
- NHS: Seasonal affective disorder (SAD)
- Wikipedia: Lux (illuminance reference values)