By 3pm the office has gone flat. Not dark, not unsafe, just dim enough that the screen is doing more of the work than it should. Nobody adjusts anything. Nobody mentions it.
There's a number for this, somewhere in UK guidance. It isn't a legal number, and it isn't about attention either.
The Law Covers Safety. It Doesn't Set a Number.
Under the Workplace (Health, Safety and Welfare) Regulations 1992, the duty is plain but unspecific: "every workplace shall have suitable and sufficient lighting," by natural light so far as reasonably practicable. No lux figure appears anywhere in the regulation itself.
The number people actually mean when they ask about office lighting law comes from the Health and Safety Executive's own supporting guidance, HSG38. It sets out a table by activity: 20 lux for a corridor, 50 lux for a construction site needing no detail perception, 100 lux for a kitchen or factory floor. Offices sit in the next band up, "work requiring perception of detail": 200 lux average, 100 lux minimum. A higher band again, 500 lux, exists, but it's for drawing offices and textile production, not ordinary desk work, something several secondary UK workplace-safety sites get backwards. HSE ties this figure to avoiding visual fatigue and letting people move around safely. Not to attention.
What One Study Found, and What It Didn't
A 2019 study from Eindhoven University of Technology sat 57 people in a simulated office and tested two lighting levels at eye level, 100 lux and 1,000 lux, against four separate cognitive tasks. Reaction speed was significantly faster at 1,000 lux in two of them. In the other two, a sustained-attention task and a working-memory task, the difference wasn't statistically significant. That's worth stating plainly rather than rounding up: light helped with some kinds of mental effort here, not all of them, and the study was run in the Netherlands, not a UK office.
A Review of 19 Studies, and One That Tested the Guidance Figure Directly
A 2021 systematic review in the Journal of Research in Health Sciences looked across 19 studies on light and attention, published between 2000 and 2019, spanning lab experiments, real workplaces, night-shift workers, and older adults. Its overall read: higher illuminance tracks with higher alertness and better attention across the studies it covers.
One study inside that review tested something closer to home. Thirty-three people, aged 19 to 26, sat through a continuous-performance test under three lighting levels: 200 lux, 500 lux, and 1,500 lux, the same 200 lux HSE recommends for an ordinary office. Attention and reaction time were significantly worse at 200 lux than at either of the higher levels, and best, 99.75% on the test's attention measure, at 1,500 lux. That original study couldn't be verified directly this session, its own publication wasn't reachable, so it's reported here as the systematic review describes it, not as independently confirmed against the source paper.
What This Doesn't Tell Us
It doesn't tell us the lux level of any specific office, or that anyone should go and buy a brighter desk lamp on the strength of two sources. It doesn't tell us anything about hydration, caffeine, or what any drink does to attention in a dim room. Neither source measured anything of the kind.
What they do say, read together, is that the guidance figure meant to stop an office being unsafe to work in and the level where attention research keeps finding its weakest scores land in the same place, 200 lux, and HSE never claimed those were the same question.
Why 0800Labs Is Interested
This is the same ground as our pieces on workplace temperature, air, and noise, looked at through a fourth window: a figure that exists for a real, stated reason, sitting right where something quieter is already happening to the people working under it. Not a time of day. Not a run of meetings. The light itself, doing something nobody in the room is measuring.
We call the occasion underneath all of this Workday Hydration. It's our own term, not an established category, and not a claim about what either source found, or about what any drink does to a dim room. It names the moment we're interested in: the one that gets built for a desk day, whatever's actually lighting it.
We're building something for that moment. If that's useful, the waitlist is below.