Light and health
Seasonal affective disorder and bright light: what the science says
As the clocks go back and the mornings get darker, many of us notice a change in how we feel. For some people this becomes seasonal affective disorder (SAD), and bright light is one of the most researched ways of supporting it.
The advice you will see everywhere is "10,000 lux for 30 minutes". It's a good starting point, but there is a lot more to the research than that, including how much light you really need, when to get it, and how the different options compare.
In this guide, we set out what SAD is, why winter affects some people so strongly, what the evidence does and doesn't show, and how to get enough bright light in practice, especially through a UK winter.
The short answer
- Does light therapy work for SAD?For many people, yes. In the largest head-to-head trial, 30 minutes of 10,000 lux light each morning worked as well as the antidepressant fluoxetine, with 67% of people responding in both groups (Lam et al., 2006). The NHS and NICE still describe the overall evidence as uncertain, mainly because many of the trials are small.
- How much light do you need?The standard is 10,000 lux at eye level for around 30 minutes. A lower brightness for longer gives comparable results, for example 2,500 lux for about two hours (Terman & Terman, 2005).
- When should you use it?In the morning, ideally within an hour or two of waking. Morning light has done better than evening light in controlled trials.
- How quickly does it work?Many people notice a difference within about a week, and the benefits tend to fade if you stop before spring.
- Who should check first?Anyone with bipolar disorder or an eye condition, or who takes medicine that increases sensitivity to light, should speak to their GP before starting.
If you are feeling very low, finding it hard to cope, or having thoughts of harming yourself, please speak to your GP, call NHS 111, or call Samaritans free on 116 123 at any time. Bright light can support your mood, but it isn't a substitute for medical care.
What is seasonal affective disorder?
Seasonal affective disorder is a type of depression that follows the seasons. For most people it starts in autumn, is at its worst through the winter months and lifts in spring.
It was first described in 1984 by Norman Rosenthal and colleagues, who also found that extending the day with bright artificial light had an antidepressant effect (Rosenthal et al., 1984).
Clinically, SAD isn't diagnosed as a separate condition. Doctors describe it as major depression "with a seasonal pattern", meaning episodes that start and end at around the same time of year, for at least two years in a row (DSM-5).
Winter SAD often looks a little different from other types of depression. Alongside low mood, people commonly describe:
- sleeping more than usual, but still feeling tired
- low energy and feeling slowed down
- a bigger appetite, particularly for carbohydrates, and weight gain
- pulling back from friends and plans
How common is SAD?
It's less common than the headlines might suggest. Studies across the US, Canada and the UK show that between 0.5% and 2.4% of people meet the full criteria at some point in their lives (Galima et al., 2020). Among people who have depression, around 10 to 20% have a seasonal pattern.
You may have seen figures suggesting one in three people in the UK have SAD. These come from surveys asking whether people feel worse in winter. That experience is very real, but it isn't the same as the clinical condition.
Winter blues or SAD?
Many more people experience a milder version, often called the winter blues: lower energy, later mornings and a flatter mood that doesn't stop you getting on with life.
| Winter blues | Seasonal affective disorder | |
|---|---|---|
| Mood | Flatter and lower than usual | Persistent low mood on most days |
| Daily life | You carry on, with more effort | Work, relationships or looking after yourself are noticeably affected |
| Pattern | Mild, most winters | A clear pattern, at least two winters in a row |
| What helps | More daylight, routine and movement | Speak to your GP; light therapy, talking therapy or medication may be offered |
The two seem to share the same underlying biology, which is why bright light comes up for both. If what you're feeling seems like more than the winter blues, it's worth speaking to your GP.
Why does winter affect mood?
The leading explanation comes down to timing. In winter, the light signal that sets our body clock each day arrives later and is much weaker, so the clock can drift out of step with the hours we keep.
Your body clock needs light every day
Our internal clock runs slightly longer than 24 hours, so it needs resetting each day. The strongest signal for this is light reaching our eyes in the morning. Specialised light-sensitive cells in the retina send this signal straight to the brain's master clock, separately from the cells we use to see.
When that morning signal is weak, the clock tends to run later. In a study of people with winter depression, most had body clocks running later than their sleep times, and the bigger the gap, the more severe their symptoms (Lewy et al., 2006). In effect, it's a mild form of jet lag that lasts all winter.
A UK winter is short on light
Source: Met Office 1991 to 2020 climate averages.
London gets around a quarter of the sunshine in December that it gets in June. On top of that, most of us spend winter days indoors, where light levels are typically around 300 lux, compared with around 5,000 lux outside on an overcast day. Our eyes adapt so well that a lit room feels bright enough, but our body clock responds to the actual amount of light, not to how bright a room looks to us.
The clocks changing can add to this. In Danish hospital records, diagnoses of depression rose by 11% in the weeks after the switch from summer time to winter time, an effect that faded over around ten weeks (Hansen et al., 2017). In the UK, the clocks go back on Sunday 25 October 2026.
Light and mood beyond SAD
The link between light and mood shows up in the wider population too. In a study of 86,772 UK Biobank participants who wore light sensors, more light during the day was associated with a lower risk of depression, and more light at night with a higher risk (Burns et al., 2023). This shows an association rather than cause and effect, but it fits with what the clinical trials show.
This isn't a failure of discipline. In large part it's a lighting problem, which means there is a lot we can do about it.
Does light therapy work for SAD?
For many people, yes. The best-designed trial found light therapy worked as well as an antidepressant, and larger reviews of the research agree that it helps. They differ on how big the effect is, mainly because many of the trials are small.
| Study | What it tested | What it found |
|---|---|---|
| Lam et al., 2006 (Can-SAD) | 96 people over 8 weeks: 10,000 lux light for 30 minutes plus a placebo pill, compared with fluoxetine plus a dim 100 lux placebo light | 67% responded in both groups, and 50% vs 54% reached remission. Light worked sooner, with fewer of some side effects |
| Golden et al., 2005 | Review of 8 controlled trials for SAD | A large effect (0.84), similar in size to antidepressant trials. Dawn simulation was also effective (0.73) |
| Pjrek et al., 2020 | Review of 19 trials with 610 patients | A smaller but significant effect (0.37), with people 1.4 times as likely to respond. The authors call for larger, better trials |
| Nussbaumer-Streit et al., 2019 (Cochrane) | Whether starting light before symptoms can prevent SAD | Only one small trial of 46 people, so too little evidence to say |
Why do the NHS and NICE describe the evidence as uncertain?
Light therapy is difficult to test in the way drugs are tested. People can see whether their lamp is bright, which makes a convincing placebo hard to design, and many studies have included only a few dozen people. For this reason, NICE advises that people who want to try light therapy should be told that "the evidence for the efficacy of light therapy is uncertain" (NICE NG222). The NHS notes that it isn't usually available on the NHS, "but many people find it helps their symptoms".
Our view is that the evidence is encouraging, though not settled. The strongest trial found light therapy matched medication, and the research has pointed in the same direction for 40 years. How large the effect is varies between studies, and light therapy won't be the right fit for everyone.
How does it work?
The most widely supported explanation is that morning light pulls a late-running body clock back in line with the day (Lewy et al., 2006). Bright light also makes us more alert within minutes, which may be why people often notice a difference before their sleep pattern has shifted. We go into the biology in more detail in our guide to how bright light therapy works.
How much light do you need for SAD?
The clinical standard is 10,000 lux at eye level for around 30 minutes each morning. What really matters, though, is the amount of light that reaches your eyes, which depends on brightness, time and distance together.
Brightness and time work together
| Brightness at eye level | Typical session | Where it comes from |
|---|---|---|
| 10,000 lux | 30-40 minutes | The modern clinical standard (Terman & Terman, 2005) |
| 2,500 lux | About 2 hours | The original 1980s approach, with comparable results (Terman & Terman, 2005) |
| 750 lux, blue-enriched | 30 minutes | One small trial of 22 people found a similar response to 10,000 lux (Meesters et al., 2011), though it was too small to be sure |
| About 300 lux (a typical room) | All day | Not enough on its own, as it's well below the levels used in trials |
A helpful way to think about this is as a daily light dose. A light box fits the dose into half an hour by being very bright. You can get a similar signal from less intense light over a longer period, which many people find easier to fit into their day. This is a rule of thumb rather than an exact formula, as very dim light doesn't add up in the same way, however long you spend in it.
Distance matters more than the number on the box
The 10,000 lux figure on a light box is measured close to the panel, usually somewhere between 20 and 60 cm away. Light drops off sharply with distance, so if you sit twice as far away you get roughly a quarter of the light. Sitting at a more natural distance, or turning your head away, can mean the amount reaching your eyes is a fraction of the number on the box.
What healthy indoor light looks like
For everyday health, rather than treatment, leading circadian scientists recommend at least 250 lux of melanopic light at eye level throughout the day (Brown et al., 2022). Melanopic light is the measure that best predicts how strongly light affects the body clock. Most homes and many offices fall well short of this, particularly in winter.
When to use light therapy, and how long it takes
Morning works best
In 1998, three controlled trials compared morning and evening light for winter depression, and morning light came out ahead in each (Terman et al., 1998; Lewy et al., 1998; Eastman et al., 1998). In one of them, 55% of people responded fully to morning light, compared with 28% to evening light and 16% to a placebo. Looking across studies, 53% of people were in remission after a week of morning light, compared with 38% with evening light and 32% with light at midday (Terman et al., 1989).
This fits with the body clock explanation, as morning light moves a late-running clock earlier. Bright light late in the evening can do the opposite and make it harder to fall asleep.
A good rule of thumb is to get your bright light within the first hour or two of waking, at the same time each day, including weekends.
How quickly does it work?
Many people notice a difference within the first week, and the improvement can continue over the following weeks (Terman & Terman, 2005). If you haven't noticed any change after a few weeks of using it consistently, speak to your GP.
How long should you continue?
Light therapy works while you are using it, rather than resetting you for the whole season. In early studies, symptoms came back when people stopped, usually within a week (Terman & Terman, 2005). Most people start in early autumn, or when their symptoms usually begin, and carry on until the brighter mornings of spring.
Sticking with it
Where light therapy most often falls down is consistency. Sitting still in front of a lamp for 30 minutes every morning for five months is a real commitment, and many people find it hard to keep up. Whichever option you choose, it's worth picking one that fits easily into your existing routine.
How do the options compare?
A 10,000 lux light box has the strongest evidence, and daylight outdoors is free and even brighter. In practice, the best option is the one you will use every morning for the whole winter.
| Option | Light at eye level | Daily effort | Evidence for SAD | Cost |
|---|---|---|---|---|
| Daylight outdoors | 5,000 lux on an overcast day, up to 100,000 lux in sunshine | A morning walk, when weather and daylight allow | Strong biology, though few trials have tested walks specifically | Free |
| Light box ("SAD lamp") | 10,000 lux at 20-60 cm | 30 minutes sitting still, facing it | The strongest: matched fluoxetine in a randomised trial (Lam et al., 2006) | About £50-£250 |
| Dawn simulator | Gradually brightens to a few hundred lux before you wake | None once it's set up | Moderate: comparable to a light box in one trial (Terman & Terman, 2006), effect size 0.73 (Golden et al., 2005) | About £50-£200 |
| Light glasses and visors | Varies, usually a few hundred lux | Worn for 20-30 minutes | Weak: a 600 lux visor did no better than dim red light (Teicher et al., 1995) | About £150-£300 |
| Whole-room bright lighting | Depends on output; Sunday Light gives 10,000 lux at 60 cm | None, as you live under it | Not yet tested in SAD trials, but built on the same dose science as light boxes | Sunday Light: £5,900 in the UK, $6,900 in the US and €6,900 in Europe |
A few practical tips:
- Check the label. The NHS recommends choosing a lamp that is 10,000 lux, UV-free and has a UKCA or CE mark.
- Place it to one side. Position a light box at eye level, slightly off to one side, at the distance the manufacturer recommends. You don't need to look directly at it, so you can read or have breakfast in front of it.
- Combine them. A morning walk on brighter days, a light box or brighter indoor light on darker ones, and a dawn simulator for the hardest mornings all work well together.
Is light therapy safe?
For most people, yes. Side effects tend to be mild and short-lived, although a few groups should speak to a doctor before starting.
Common side effects include headaches, eye strain, nausea and feeling jittery, reported by around 6 to 16% of people in trials. These usually ease within the first few days, or by sitting a little further back or shortening the session (Terman & Terman, 2005).
Speak to your GP first if you:
- have bipolar disorder, as bright light can trigger hypomania or mixed states
- have an eye condition, such as macular degeneration, glaucoma or diabetic retinopathy
- take medicine that makes you more sensitive to light, for example some antibiotics, acne treatments or herbal remedies such as St John's wort
- are already being treated for depression, so that light therapy can be added safely alongside your treatment
It's also best to avoid bright light late in the evening, as it can delay sleep.
Does vitamin D help with SAD?
On its own, probably not, and it's a separate question from light therapy. In the UK, the sun is too low from around October to March for our skin to make vitamin D (Webb et al., 2018; Kift et al., 2018), which is why the NHS advises everyone to consider taking 10 micrograms a day through autumn and winter. That's sensible general advice.
For SAD specifically, the evidence is weak. A 2020 review notes that vitamin D "is not well studied in SAD" (Galima et al., 2020), and a randomised trial of a high daily dose found no effect on symptoms (Frandsen et al., 2014).
Light therapy also works through the eyes rather than the skin. Light boxes, and lighting such as Sunday Light, are designed to be UV-free, so they don't produce vitamin D. Vitamin D supports your bones and muscles, whilst bright light supports your body clock.
A winter light plan for the UK
Here is how we would approach the darker months, especially if you know winter tends to be hard for you.
- Start early. Begin in late September or early October, before the clocks go back, rather than waiting until you feel low.
- Get bright light every morning. Aim for bright light within an hour or two of waking, at the same time each day, including weekends.
- Go outside when you can. Even an overcast morning gives you around 5,000 lux, many times brighter than indoors. Ten to twenty minutes on the walk to work or with your morning coffee all counts.
- Bring brighter light indoors on dark days. On mornings when the sun rises after you've started your day, use a light box or brighter indoor lighting in the room where you already spend your mornings.
- Keep your evenings dim. Warmer, dimmer light in the two to three hours before bed helps protect your sleep.
- Keep going until spring. Stopping early is the most common reason the benefits fade.
- Check in with yourself. If your mood is still low after a few weeks, or getting worse, speak to your GP.
Where Sunday Light fits
Sunday Light isn't a medical device, and we don't claim that it treats SAD. We built it because of step 4.
Bright light is well researched, but in our experience the routine is where it tends to fall down. Sitting close to a light box for half an hour every morning, for five months of the year, is a lot to ask. We wanted to bring the light to you instead, by filling the room you already spend your mornings in.
Sunday Light hangs from the ceiling like a pendant and produces 34,500 lumens, around 10,000 lux at 60 cm, spread across a large, soft panel. It is UV-free, has a colour rendering index of 97+, and shifts automatically from bright, cool light in the morning to warm, dim light in the evening. Rather than sitting in front of it, you can have breakfast, work or read underneath it, and get the benefits of bright light as part of your normal day.
"It produces a level and quality of light that approximates sunlight, yet its design allows prolonged use that is both comfortable and convenient. Critically, you do not have to sit next to the device to get the levels of light that will deliver physiological benefits."
Professor Russell Foster, Director of the Sleep and Circadian Neuroscience Institute, University of Oxford, and adviser to Sunday Light
If a light box works for you, it's an affordable and well-evidenced place to start. If you know you're unlikely to sit in front of one every morning, you can read more about how Sunday Light compares with SAD lamps.
Discover Sunday LightFrequently asked questions about light therapy for SAD
Does light therapy work for seasonal affective disorder?
For many people, yes. In the largest head-to-head trial, 30 minutes of 10,000 lux light each morning worked as well as the antidepressant fluoxetine, with 67% of people responding to each. Pooled analyses agree it helps, though NICE and the NHS describe the overall evidence as uncertain because many trials are small.
How long should I use a SAD lamp each day?
About 30 minutes at 10,000 lux, or around two hours at 2,500 lux. Sit at the distance the manufacturer specifies, usually 20 to 60 cm, because the dose drops sharply as you move away.
What time of day should I use light therapy?
In the morning, within an hour or two of waking. Controlled trials found morning light more effective than evening light, and bright light late in the evening can make it harder to sleep.
How long does light therapy take to work for SAD?
Many people notice a difference within a week. Keep using it daily through the winter; symptoms often return within days to weeks if you stop before spring.
Can ordinary indoor lighting help with SAD?
Not at normal levels. Typical rooms give around 300 lux, far below the 2,500 to 10,000 lux used in trials. Much brighter whole-room lighting can raise the dose, but it hasn't yet been tested in SAD trials.
Is going outside better than a light box?
Outdoor light is brighter, even on a cloudy day (around 5,000 lux), and is free. The problem in a UK winter is timing: on many working days the sun rises late and sets early. Most people do best combining both.
Can light therapy prevent SAD?
Possibly, but it hasn't been properly tested. A Cochrane review found only one small trial of starting light before symptoms, too little to say. Many people with a clear seasonal pattern choose to start in early autumn anyway.
Do SAD lamps give you vitamin D?
No. Light boxes are UV-free and work through the eyes, not the skin. For vitamin D, the NHS advises considering a 10 microgram daily supplement from autumn to spring.
Who shouldn't use light therapy without advice?
People with bipolar disorder, eye conditions such as macular degeneration or glaucoma, or who take medicines that increase sensitivity to light. Speak to your GP first.
What is the difference between SAD and the winter blues?
The winter blues are a milder dip in mood and energy that doesn't stop you living normally. SAD is clinical depression that returns at the same time each year, for at least two years running, and affects daily life.
References
- Brown, T.M. et al. (2022). Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology, 20(3), e3001571. Link
- Burns, A.C. et al. (2023). Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people. Nature Mental Health, 1(11), 853-862.
- Eastman, C.I. et al. (1998). Bright light treatment of winter depression: a placebo-controlled trial. Archives of General Psychiatry, 55(10), 883-889.
- Frandsen, T.B. et al. (2014). Vitamin D supplementation for treatment of seasonal affective symptoms in healthcare professionals: a double-blind randomised placebo-controlled trial. BMC Research Notes, 7, 528. Link
- Galima, S.V., Vogel, S.R. & Kowalski, A.W. (2020). Seasonal affective disorder: common questions and answers. American Family Physician, 102(11), 668-672. Link
- Golden, R.N. et al. (2005). The efficacy of light therapy in the treatment of mood disorders: a review and meta-analysis of the evidence. American Journal of Psychiatry, 162(4), 656-662. Link
- Hansen, B.T. et al. (2017). Daylight savings time transitions and the incidence rate of unipolar depressive episodes. Epidemiology, 28(3), 346-353.
- Kift, R. et al. (2018). Is sunlight exposure enough to avoid wintertime vitamin D deficiency in United Kingdom population groups? International Journal of Environmental Research and Public Health, 15(8), 1624.
- Lam, R.W. et al. (2006). The Can-SAD study: a randomized controlled trial of the effectiveness of light therapy and fluoxetine in patients with winter seasonal affective disorder. American Journal of Psychiatry, 163(5), 805-812. Link
- Lewy, A.J. et al. (1998). Morning vs evening light treatment of patients with winter depression. Archives of General Psychiatry, 55(10), 890-896.
- Lewy, A.J. et al. (2006). The circadian basis of winter depression. PNAS, 103(19), 7414-7419.
- Meesters, Y. et al. (2011). Low-intensity blue-enriched white light (750 lux) and standard bright light (10,000 lux) are equally effective in treating SAD. BMC Psychiatry, 11, 17.
- Met Office. Heathrow climate averages, 1991-2020. Link
- NHS. Seasonal affective disorder (SAD): treatment. Link
- NICE (2022). Depression in adults: treatment and management (NG222), recommendation 1.4.39.
- Nussbaumer-Streit, B. et al. (2019). Light therapy for preventing seasonal affective disorder. Cochrane Database of Systematic Reviews, CD011269.
- Pjrek, E. et al. (2020). The efficacy of light therapy in the treatment of seasonal affective disorder: a meta-analysis of randomized controlled trials. Psychotherapy and Psychosomatics, 89(1), 17-24.
- 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.
- Teicher, M.H. et al. (1995). The phototherapy light visor: more to it than meets the eye. American Journal of Psychiatry, 152(8), 1197-1202.
- Terman, M. et al. (1989). Light therapy for seasonal affective disorder: a review of efficacy. Neuropsychopharmacology, 2(1), 1-22.
- Terman, M. & Terman, J.S. (2005). Light therapy for seasonal and nonseasonal depression: efficacy, protocol, safety, and side effects. CNS Spectrums, 10(8), 647-663.
- Terman, M. & Terman, J.S. (2006). Controlled trial of naturalistic dawn simulation and negative air ionization for seasonal affective disorder. American Journal of Psychiatry, 163(12), 2126-2133.
- Terman, M., Terman, J.S. & Ross, D.C. (1998). A controlled trial of timed bright light and negative air ionization for treatment of winter depression. Archives of General Psychiatry, 55(10), 875-882.
- Webb, A.R. et al. (2018). Colour counts: sunlight and skin type as drivers of vitamin D deficiency at UK latitudes. Nutrients, 10(4), 457. Link