Every autumn, a predictable pattern plays out in UK workplaces. Energy levels dip. Concentration becomes harder to sustain. Absenteeism climbs. For most managers and facilities teams, this is written off as the inevitable cost of winter. But a significant body of research suggests that a meaningful part of this seasonal toll is not inevitable at all, and that how a workplace is lit during the darker months plays a real role in whether people can function at their best.
This post looks at Seasonal Affective Disorder, what the science actually says, how it maps onto the UK workforce, and what the evidence tells us about lighting's role in prevention and management. It is the fourth post in our series on Lighting, Health and the Human Mind.
Seasonal Affective Disorder was first formally identified and named by Norman Rosenthal and his colleagues at the US National Institute of Mental Health in 1984. Their study, published in the Archives of General Psychiatry, described a pattern of recurrent winter depression that lifted in spring and summer, and demonstrated that extending the photoperiod using bright artificial light had a measurable antidepressant effect. It has since become one of the most-cited papers in psychiatry, with over 3,400 citations to date.
SAD is classified as a subtype of major depressive disorder or bipolar disorder with a seasonal pattern. Its core symptoms include persistent low mood, loss of energy, hypersomnia, carbohydrate craving, weight gain, social withdrawal, and impaired concentration. These symptoms typically emerge in September or October, peak between November and February, and resolve in spring. They are not mild inconveniences. For people with clinical SAD, the condition can make it very difficult to function in a normal working day.
The leading mechanistic theory involves the effect of reduced light exposure on two intersecting systems: the suprachiasmatic nucleus, which governs circadian rhythm, and the serotonergic system, which is central to mood regulation. Shortened winter days in higher-latitude countries like the UK mean that many people are commuting in the dark both morning and evening, spending their entire working day under artificial light, and receiving far less light signal to the ipRGCs (intrinsically photosensitive retinal ganglion cells) that regulate circadian entrainment.
The UK is particularly exposed to seasonal light deprivation. At the latitude of Edinburgh or Manchester, winter day length falls to around seven or eight hours, and even that light is often weak, overcast, and filtered further by commuting and indoor working patterns.
The figures on prevalence vary depending on the diagnostic threshold. The Royal College of Psychiatrists estimates around 3 in every 100 people in the UK experience significant winter depression meeting clinical SAD criteria. A 2015 systematic review published in Depression Research and Treatment puts the UK figure at approximately 2% for clinical SAD. The more important number for employers, however, is the rate of subsyndromal SAD, sometimes called the winter blues, which is estimated to affect around 20% of the UK population. A 2025 meta-analysis in Science Direct examining global SAD prevalence by latitude found that subsyndromal SAD affects an estimated 14.3% of women in the United Kingdom specifically. The Onebright clinical organisation cites around 20% of people in the UK experiencing mildly debilitating depressive symptoms in winter.
In a typical office of 50 people, that means somewhere between one and ten people may be experiencing a meaningful winter-related mood and energy impairment at any point between October and March. The effects on concentration, output, decision-making, and interpersonal functioning are real even if they are hard to measure at an individual level.
The broader context is a UK workforce already under considerable mental health pressure. The ONS reported that 148.8 million working days were lost to sickness or injury in 2025, with stress, depression, and anxiety accounting for a large proportion of that total. UK workers took an average of 5.7 days off due to poor mental health in 2024, at an estimated cost to employers of £13.9 billion. The government's Keep Britain Working review estimated the annual cost to employers of poor workplace health at around £85 billion.
Seasonal dips in mental health contribute to this picture. They are not the only factor, but they are a recurring, predictable one, and predictable problems are, in principle, addressable ones.
The evidence base for light therapy in treating clinical SAD is robust by the standards of psychiatric intervention. Terman and Terman's 2005 review in CNS Spectrums, one of the most widely cited summaries of the evidence, concluded that bright light therapy administered at the right time and in the right dose is the most efficient, tested, and safe treatment for SAD. Standard treatment protocol involves 30 minutes of 10,000 lux broad-spectrum white light in the early morning, which suppresses melatonin and advances the circadian phase. Studies consistently show remission rates of around 50% with standard protocol, rising to approximately 80% when treatment is timed to the individual's circadian phase.
It is important to be clear about what this evidence does and does not support. Clinical light therapy, using certified lightboxes at 10,000 lux, has a strong and replicated evidence base for treating diagnosed SAD. The evidence for general workplace lighting as a preventive or therapeutic intervention at population level is more complex and still developing. It would be misleading to imply that upgrading office lighting cures or prevents SAD in the same way that clinical light therapy does.
What the evidence does support more clearly is that access to adequate light during the working day matters for circadian health, mood, and alertness in the broader population, including those experiencing subsyndromal symptoms. A 2014 study by Boubekri et al., published in the Journal of Clinical Sleep Medicine, found that office workers with greater light exposure had longer sleep duration, better sleep quality, more physical activity, and higher quality-of-life scores compared with those in windowless environments. A 2025 long-term study of circadian-based lighting strategies in real office conditions, published in Science Direct, found that fixed and dynamic lighting protocols aligned with circadian principles produced measurable improvements in sleep quality and circadian regulation, while lighting that suppressed natural melatonin timing had a measurably negative effect.
The relevant light metric for circadian and mood-related outcomes is not photopic lux alone. The ipRGCs that drive circadian entrainment are most sensitive to short-wavelength blue light, which is captured by the melanopic equivalent daylight illuminance (melanopic EDI) metric.
The WELL Building Standard v2 sets a target of 250 equivalent melanopic lux (EML), measured vertically at eye level at 1.2m, at 75% of workstations for at least four hours per day throughout the year. A 2022 consensus paper by Brown et al. in PLOS Biology, representing a coalition of sleep and circadian researchers, recommended a minimum melanopic EDI of 250 lux throughout the daytime as a guideline for healthy indoor light exposure.
A 2025 UCL study assessing melanopic equivalent daylight illuminance in real UK office environments found that daylight may be sufficient for circadian entrainment in spaces that meet the high level of the EN 17037 daylighting standard with unobstructed windows. In many actual office environments, particularly those with deep-plan floorplates, poor window access, or heavily glazed facades that filter short-wavelength light, the melanopic content of available light may fall well below what circadian research suggests is optimal. Electric lighting in many standard office installations does not compensate for this deficit, because most conventional lamps are selected and specified against photopic lux targets alone.
There are several practical implications for designers and employers working in UK commercial environments:
It is worth being honest about the limits of what we know. The strongest evidence for light and mood relates to clinical SAD treatment using certified light therapy devices. The evidence for circadian-tuned office lighting improving mood or productivity outcomes in the broader working population is encouraging but less definitive. Most relevant studies are relatively short in duration, conducted in controlled conditions that do not always reflect real-world variability, or focused on sleep outcomes rather than mood or productivity directly.
That does not mean the design direction is wrong. The circadian evidence base is growing rapidly, and the consensus among sleep researchers and circadian biologists has moved clearly towards the view that the melanopic content and timing of daytime light exposure matters for health. What it does mean is that claims about lighting design solving seasonal mood problems in the workforce should be made carefully and proportionately.
A well-designed, daylight-connected, circadian-aware lighting scheme will support the conditions in which people are less likely to experience the worst effects of light deprivation in winter. It is one part of a wider picture that also includes building design, HR policy, occupational health provision, and individual clinical care where needed.
This post sits within a broader discussion of how light affects the human mind and body in the workplace. Related posts in the series cover the circadian evidence base in detail, the neuroscience of light and alertness, and the specific needs of neurodivergent employees. For the specification foundations that underpin the choices discussed here, see also our Office Lighting Design Guide and our article on What Is UGR in Lighting?
At 299 Lighting, we work with designers, employers, and facilities teams who are trying to understand what genuinely evidence-based lighting specification looks like in practice. If you are working on an office project where seasonal wellbeing, circadian performance, or employee health outcomes are part of the brief, we are happy to discuss what the current evidence supports and how to build it into a workable specification.