Body clock & sleep
Your body runs on an internal clock. Daylight is what keeps it on time, and keeps it strong.
THE BIG IDEA
A strong rhythm organises everything
Almost every tissue in your body follows a daily rhythm, and a master clock in the brain keeps them in time. Daylight is its main time signal. Circadian scientist Sean Cain explains that it isn’t only morning light that matters. Whenever the sun is out, daylight strengthens the clock’s rhythm, its ‘amplitude’. Too little daylight and too much light at night weaken it, leaving your body less sure what time it is.
HOW IT WORKS
How daylight sets your clock
1
Your clock runs a little off
For most people, especially men, the internal day runs slightly longer than 24 hours, so it needs resetting every day.
2
Morning light nudges it forward
Light early in the day shifts the clock earlier, helping your internal day line up with the real one.
3
Daylight makes it stronger
Bright light through the day boosts the rhythm’s strength, while light at night blunts it and suppresses melatonin.
WHAT THE SCIENCE SAYS
The numbers behind the idea
1 month
How long author Rowan Jacobsen lived without artificial light at night in the Arizona desert. Dark nights alone didn’t cure his insomnia; bright days plus dark nights did.
10,000 lux
The morning brightness Sarah Kleiner aims for. Indoors is often only 100–300 lux, while outside can reach 100,000 lux or more.
50%
Evening light in typical Melbourne homes was bright enough to cut melatonin by about half, right up to bedtime.
13%
Rise in heart rate variability when Shadowmap’s CEO Georg watched every sunrise for three weeks. His resting heart rate fell 8%.
FROM THE PODCAST
In their own words
Prof. Sean Cain
Circadian researcher
Sarah Kleiner
Circadian health coach
Rowan Jacobsen
Author
WHAT YOU CAN DO
Keep your clock on time
Get outside early
Step outside within an hour of waking. If you miss it, don’t worry: daylight later in the day still helps.
Make the most of the day
Take breaks outdoors and work near a window. Foggy mid-afternoon? Coach David Herrera’s fix is a glass of water and five minutes outside.
Keep evenings dim
Use warm, low lights after sunset and avoid bright white overhead lights close to bedtime.
Plan with the Sun when you travel
Seeking or avoiding light at the right times speeds up recovery from jet lag. Sometimes that means sunglasses on arrival.
Put it into practice
Find your sunrise
Shadowmap shows exactly when the sun rises over your street, garden or local park, so you know where to catch the first light.
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 →
Sun, skin & safety
What the research says about melanin, building up exposure gradually and never burning.
Read the guide →
Sources & further listening
Prof. Sean Cain, research on circadian amplitude, evening light at home and jet lag. Georg Molzer, Shadowmap sunrise experiment. Sunlight Matters podcast episodes with Sean Cain, Sarah Kleiner, Rowan Jacobsen, David Herrera and Georg Molzer. This page is general information, not medical advice.
