Indoor light
In a single lifetime, the light inside our homes, schools and offices changed completely. Our biology didn’t.
THE BIG IDEA
We edited the Sun out of our buildings
For generations, indoor light came from fire and then incandescent bulbs, both rich in warm, infrared light much like sunlight. Then, in the name of energy efficiency, incandescent bulbs were phased out and replaced with LEDs designed only for what our eyes can see. Add modern glazing that blocks infrared, and the fact that we spend around 90% of our time indoors, and many of us now live almost entirely without the long wavelengths our bodies evolved with.
WHAT CHANGED
How indoor light lost its infrared
1
Incandescent bulbs were banned
Judged on light output per watt alone, with no assessment of the effect on people or the environment, broad-spectrum bulbs were phased out around the world.
2
LEDs filled the gap
Efficient and bright, but most are blue-enriched and produce almost no infrared, and they’re now used everywhere, often right up to bedtime.
3
Building windows started filtering too
Energy-saving double and triple glazing keeps heat in by blocking infrared, so even the daylight that reaches us indoors has lost much of it.
FROM THE PODCAST
In their own words
Anna Levin
Author of ‘Incandescent’
Scott Zimmerman
Lighting engineer
Sarah Kleiner
Circadian health coach
WHAT THE SCIENCE SAYS
The numbers behind the idea
From the lighting engineers, scientists and doctors we’ve spoken to on Sunlight Matters.
400–700 nm
The narrow band modern lighting is designed around: what our eyes see. Sunlight stretches from about 280 nm to beyond 35,000 nm.
~0
How much infrared you get indoors with LED lighting and modern glazed windows, according to circadian researcher Sean Cain.
Everyone
Whose vision improved when Glen Jeffery’s team swapped LEDs for incandescent bulbs in a windowless UCL office.
30%
How much faster intensive care patients were discharged in a trial of infrared light, a sign of what’s missing indoors.
WHAT YOU CAN DO
Bring better light indoors
Add a broad-spectrum lamp
Keep an incandescent or halogen lamp alongside your LEDs, especially for evenings and reading.
Open the window
Glass filters infrared; an open window doesn’t. Work by an open window when the weather allows.
Dim the lights at night
Use dimmer switches and warm bulbs after sunset, and turn off the lights you don’t need.
Take your breaks outside
No indoor light matches the sun. A few minutes outdoors gives you the full spectrum.
Put it into practice
See the sun inside your building
With Shadowmap Studio, architects and developers can model a building and see when direct sunlight reaches each façade and window through the day and the year.
KEEP READING
More health guides
Explore how light shapes other parts of your health, from your cells to your sleep.
Light & your cells
How infrared light from the sun helps the energy factories in your cells run more smoothly.
Read the guide →
Light & mood
Why bright days and dark nights are linked to a lower risk of depression.
Read the guide →
Body clock & sleep
How morning light, dark evenings and a strong daily rhythm keep your body in sync.
Read the guide →
Sources & further listening
Sunlight Matters podcast episodes with Anna Levin, author of ‘Incandescent’, and with Scott Zimmerman, Sarah Kleiner, Roger Seheult, Glen Jeffery and Sean Cain. Fosbury R., Seheult R., Zimmerman S. & Jeffery G., ‘Editing the Sun: how indoor light lost touch with biology’ (2026). This page is general information, not medical advice.
