TempoLife › Features › The dark season: light, sleep and vitamin D › outdoors
The daylight arithmetic: how little light we actually get
Indoors is a cave even at noon in summer; outdoors on a winter day beats any lamp. The lux numbers explain why.
This is seasonal self-care information, not a diagnosis and not treatment advice. Seasonal affective disorder is a clinical diagnosis made by a clinician; bright-light therapy is not safe to self-start for everyone (bipolar disorder and some eye conditions come first); and any measured vitamin D deficiency is treated at a clinician's dose, not a supplement-shelf one. If thoughts of self-harm are present, the step is crisis services now — the <a href="/therapists">mental health page</a> carries the national crisis lines.
The numbers
Daylight outdoors runs from about 1 000 lux on a heavily overcast winter day to over 100 000 lux in summer sun. A bright office is 300–500 lux; a living room in the evening is 50–150; the light box that feels painfully bright is 10 000 lux for the few centimetres in front of it. The practical reading: half an hour outdoors on a dull December noon delivers more light to the eye than a day indoors under electric light — which is why the cheapest light therapy in this region is going outside.
Cloud is not darkness
An overcast winter sky is not dim in circadian terms: the diffuse light is still thousands of lux, far above indoor levels, and the eye's melanopsin system responds to it. The common mistake is judging outdoor light by mood — "grey" feels dark — when the photometer and the biology say otherwise.
Windows and glass
Glass blocks the UV that makes vitamin D but passes most visible light, so a bright window seat is real light therapy while it lasts. The catch is geometry: light falls off with the square of distance, and a metre from the glass the dose has collapsed. Sitting at a window, not across the room from one, is the difference.
Making the outdoor dose routine
The mechanism works on minutes, not heroics: the walk to the shop, the school run, lunch outside, shovelling snow, the dog. Ten to thirty minutes of outdoor light in the morning half of the day is the dose the biology responds to, and it is the one intervention on this page that costs nothing and cannot be overdosed.
Light therapy dosing: the bright-light trials (10 000 lux, 30 minutes, morning). Vitamin D: the Nordic national nutrition recommendations. Circadian facts: the standard chronobiology literature (melanopic response, dim-light melatonin onset). The depression line: clinical diagnostic practice — seasonal affective disorder is diagnosed by a clinician. Checked 2026-09-06.
Frequently asked questions
Does outdoor light work on cloudy days?
Yes — an overcast day is still thousands of lux, which is ten to a hundred times any indoor lighting, and the circadian system responds to it. The grey look is misleading; the light is there.
Is sitting by a window enough?
Only if you are actually at the glass: visible light passes through, but intensity falls with distance squared, so a metre away the dose is a fraction. A window seat is real light therapy; a window across the room is a view.
The rest of the dark season
The dark season
All six winter pages, with the numbers behind them.
Light therapy: the dose that works
10 000 lux for 30 minutes in the morning — the dose the trials actually used, and how to deliver it.
Vitamin D in the dark season
What latitude does to skin synthesis, and what the Nordic recommendations actually say about supplements.
Winter sleep: longer, later and how to steer it
The dark season pulls sleep later and longer; the lever that fixes it is morning light, not evening discipline.
Winter blues and when it is depression
The difference between the seasonal dip and seasonal affective disorder, and the line where help is needed.
Track the season, not just the symptoms
TempoLife keeps sleep, steps and daylight in one picture, so the difference between your November and your June is on the chart instead of in your memory.