A cloudy morning outside beats a bright office for your body clock

Key takeaways

• Outdoor light typically runs about 1,000 to 10,000 lux on a cloudy day and 50,000 lux or more in sunshine, while a typical office is around 500 lux⁴⁶.
• In a classic 2003 lab study, bright light after the body’s nightly temperature low moved the clock earlier, while light before it moved the clock later⁷.
• In a 2023 study of 86,772 adults who wore light sensors, people with the brightest days had about 19% lower odds of major depression, though the study can’t prove light was the cause¹³.
• In a 2025 analysis of 11 trials, people with nonseasonal depression who used a 10,000-lux light box reached remission more often, 40.7% versus 23.5%⁸.
• If you have bipolar disorder, an eye disease or take a medicine that raises light sensitivity, talk with a clinician before using a light box⁹¹⁰.

Between 2013 and 2015, tens of thousands of adults in the United Kingdom wore a wrist sensor for a week. It logged their movement and, more unusually, how much light reached them³.

Later, researchers including Sean Cain of Monash University in Australia compared those light logs from 86,772 people with their mental health¹². People in the top quarter for daytime light had about 19% lower odds of major depression than people in the bottom quarter³. People with the most light at night had about 30% higher odds³.

That kind of study can’t prove that light changed anyone’s mood. Depressed people may simply spend less time outside. But it lines up with lab work, described below, showing that light at the right time of day sets the body clock.

“It’s about getting bright light in the day and darkness at night,” Cain said². The brightest light you can get is free, and it’s outside your door. This is how morning light works, how much you need, when a light box makes sense, and how to rate the advice you’ve heard.

Light’s timing decides which way your clock moves

For most people, the body’s master clock runs a little longer than 24 hours, so it needs light each day to stay on time¹². Our article on circadian rhythms explains how the clock itself works. This one is about using light on purpose.

The key idea is that the same light can push your clock in opposite directions, depending on when it hits your eyes. Scientists map this with a phase response curve, a chart of how much and which way the clock shifts when light arrives at each hour.

In a 2003 study in the Journal of Physiology, researchers kept 21 healthy adults, ages 19 to 44, in dim light, then gave each one a single 6.7-hour dose of very bright light, about 10,000 lux, at different times of day or night⁷. They tracked the shift by measuring melatonin, the clock’s evening hormone⁷.

Light timed before the low point of body temperature, which falls in the night, moved the clock later⁷. Light after that low point moved it earlier⁷. The switch between the two happened fast⁷.

In that study, the temperature low came about two hours before people’s usual wake time, by our math from the reported averages⁷. For most people, then, light soon after waking falls on the “earlier” side of the curve. Bright light late in the evening falls on the “later” side.

The shifts could be large. In individual volunteers, the clock moved as much as 3.6 hours later or 2 hours earlier⁷. But that took hours of light as bright as a bright cloudy day, far more than most people get. Everyday doses are smaller, so a steady daily habit matters more than any single morning.

Outdoors on a gray day still beats most indoor rooms

Brightness is measured in lux, the amount of light falling on a surface. The numbers surprise most people.

Setting	Typical light
Public stairwell	About 200 lux6
Office	About 500 lux6
Supermarket	About 750 lux6
Outdoors, cloudy day	About 1,000 to 10,000 lux4
Outdoors, sunny day	50,000 lux or more4

These are typical figures, not fixed values; real levels swing with weather, season and where you stand. Even so, a gray morning outdoors can be several times brighter than an office, and a sunny one 100 times brighter or more. Your eyes adjust, so indoor rooms seem bright enough. The numbers show how big the gap is.

In 2022, 18 light and sleep scientists published consensus recommendations in the journal PLoS Biology⁵. They set their targets in “melanopic” light, a measure of how strongly light stimulates melanopsin, a light-sensing pigment in retinal cells that signal the body clock⁵.

Their targets, as reported by the science site Inverse: at least 250 melanopic lux during the day, no more than 10 in the three hours before bed, and darkness, ideally 1 lux or less, while sleeping⁶. Melanopic lux isn’t quite the same as ordinary lux, so these numbers don’t line up one-to-one with the table above⁶.

One author, Timothy Brown of the University of Manchester, offered a simple rule of thumb: cooler white light during the day and warmer white light in the evening⁶.

Rating the “10 minutes of morning sun, no sunglasses” rule

A version of this advice is popular on podcasts and social media: get outside soon after waking for about 10 minutes, longer when it’s cloudy, and skip the sunglasses. This is how each piece holds up.

Piece of the advice	What the research shows	Rating
Morning light moves your clock earlier	Shown in a classic lab study of the phase response curve, in line with expert consensus that light is the clock’s main signal57	Strong evidence
Outdoor light beats indoor light	Measured lux levels differ many times over46	Strong evidence
Daylight protects your mood	A huge sensor study found a link, but it can’t show cause13	Promising but early
Exactly 10 minutes is enough	We found no trial testing that dose; a federal guide suggests at least 30 minutes of sunlight a day11	Not enough evidence
Take off your sunglasses	We found no study testing sunglasses on versus off	Not enough evidence
Look toward the sun	Light only needs to reach your eyes; Mayo Clinic says not to look straight into a light box9	Evidence says no, or unsafe

The gist is sound. The precise numbers are guesses dressed up as rules.

The National Heart, Lung, and Blood Institute (NHLBI) is more generous with time. Its sleep guide suggests trying for at least 30 minutes of natural sunlight a day. If you have trouble falling asleep, it suggests about an hour of morning sunlight and dimmer lights before bed¹¹.

Never stare at the sun. If your eye doctor has told you to wear sunglasses, keep wearing them and spend a little longer outside instead.

Light boxes help depression, with real cautions

A light box is a lamp that delivers very bright light with most ultraviolet light filtered out, typically 10,000 lux, about as bright as a bright cloudy day⁴⁹. The usual routine is 20 to 30 minutes, early in the morning, with the box 16 to 24 inches from your face⁹.

Seasonal depression. Light therapy is well established for seasonal depression, per the American Psychiatric Association, which calls it a long-standing key treatment⁴. Most people who use it start in early fall and continue until spring⁹. Mayo Clinic says it probably won’t cure the condition but may ease symptoms and raise energy, sometimes within days and sometimes after two weeks or more⁹.

Depression that isn’t seasonal. In a 2025 JAMA Psychiatry analysis, researchers pooled 11 randomized trials with 858 adults⁸. In randomized trials, people are assigned by chance to one treatment or another. Light boxes of 10,000 lux used at least 30 minutes a day were compared with a placebo, dim red light or an antidepressant alone⁸.

Remission, meaning symptoms mostly gone, came for 40.7% of people using bright light versus 23.5% of the others⁸. All 11 trials were rated at low risk of bias, though the AAFP summary lists the funding as unknown⁸. Some trials added light to an antidepressant rather than replacing it⁸.

Who should be careful. Side effects are usually mild and brief: tired eyes, headaches, queasiness or feeling on edge⁹. Bright light can tip some people with bipolar disorder into mania, so if you have it, plan any use with your prescriber⁹¹⁰. Have an eye doctor weigh in first if you’ve had glaucoma, cataracts or eye damage from diabetes¹⁰. Ask your clinician, too, if lupus or another condition makes your skin react to light, or if any of your medicines carry a sun-sensitivity warning. Mayo Clinic’s examples include some antibiotics and anti-inflammatories, plus the herb St. John’s wort⁹.

What the label doesn’t tell you. The FDA doesn’t regulate light boxes for treating seasonal depression, you don’t need a prescription, and insurance rarely pays⁹¹⁰. Tanning beds are no substitute, Mayo Clinic warns: their UV light can damage skin and sharply raise skin cancer risk⁹.

Older adults often need more daylight, not less

Body clocks change with age. NHLBI notes that the rhythm and timing of the body’s clocks decline as people get older, that less activity and less time outdoors affect rhythms, and that older adults tend to wake earlier¹².

A Dutch trial tested what more light could do. From 1999 to 2004, researchers studied 189 residents of 12 group care homes, with an average age of almost 86; most had dementia¹³.

Six of the homes got ceiling lights of about 1,000 lux, left on from about 9 in the morning until 6 in the evening. The other homes kept dimmer lighting of about 300 lux¹³¹⁴.

Over an average of 15 months, residents in the brighter homes had 5% less mental decline, 19% lower depression scores and a 53% slower yearly rise in everyday limitations, all relative figures¹³. The authors called the benefit modest and reported no harm from the all-day light¹³¹⁴. They also found that melatonin pills alone worsened mood, a problem that faded when melatonin was paired with light¹³.

If you care for an older parent, light is an easy thing to check. Seat them by a window for breakfast, plan a morning walk or porch sit, and ask the care home how bright its shared rooms are.

Your daily “light diet”

Think of light as something you take in all day, with a different goal for each part of the day.

Time	Goal	How to do it	Why
First hour after waking	Bright	Step outside, walk the dog, or eat breakfast by a window	Morning light moves the clock earlier7
Daytime	Stay bright	Work near a window and take breaks outdoors; aim for 30 minutes of sunlight or more	Federal guidance on daily sunlight11; brighter days tracked with lower depression odds3
Last 3 hours before bed	Dim and warm	Use lamps instead of overhead lights, with warm-white bulbs	Consensus target of 10 melanopic lux or less6
Sleep	Dark	Lights off; cover glowing devices	Night light tracked with higher depression odds3

On dark winter mornings, get outside as soon as it’s light, even midmorning. If winter blues are a yearly pattern, ask a clinician about a light box. Mayo Clinic suggests starting in early fall and continuing into spring⁹. Later articles in this series cover screens and bedroom darkness in more detail.

How strong is the evidence?

Claim	Evidence level	What it rests on
Morning light shifts the clock earlier, and late-evening light shifts it later	Strong evidence	A controlled lab study mapping the phase response curve7, consistent with a 2022 expert consensus5
A light box helps nonseasonal depression, alone or as an add-on	Strong evidence	11 randomized trials at low risk of bias8
More daytime light protects mental health in daily life	Promising but early	One very large study at a single point in time13
Brighter care-home lighting slows decline in dementia	Promising but early	One long randomized trial with modest effects13
A set number of minutes of morning sun is required	Not enough evidence	We found no trial of a specific dose; federal advice is general11

We left seasonal depression off this scale on purpose. The APA calls light therapy a long-standing key treatment for it⁴, but we couldn’t open a review of the seasonal trials to rate them ourselves.

Sunlight is the original light therapy

Many natural-health readers already believe in time outdoors, early rising and living by the seasons. On light, the science largely agrees. The strongest clock signal you can get costs nothing.

It also comes with a bonus that a lamp doesn’t offer. A morning walk adds movement, fresh air and, often, company. And light’s clock and mood effects work through your eyes, not your skin, so you don’t need to bare skin or risk a sunburn⁹¹².

Apply the same skepticism you’d bring to any product. A light box has real trials behind it for depression⁸. A pricey “circadian” gadget may not. Ask what it was tested against, and whether the light reaching your eyes matches the research.

When to get help

Light can support mood and sleep, but it doesn’t replace care. Mayo Clinic advises talking with a doctor or mental health provider before starting light therapy, and seeking help right away if you’re worried about how it is affecting your mood or thoughts⁹. If you have thoughts of harming yourself, call or text the 988 Suicide & Crisis Lifeline right away.

Don’t stop an antidepressant to try light instead. Mayo Clinic lists mania and hyperactivity among possible side effects in people with bipolar disorder, so if you feel unusually wired while using a light box, call your provider⁹.

See a clinician for sleep, too, if a bed partner hears loud snoring, gasping or pauses in breathing, or if you’re very sleepy in the daytime¹¹. Get checked for evening leg discomfort that eases when you move, and for acting out your dreams¹¹. If you’re fighting to keep your eyes open while driving, pull over¹¹. Later articles in this series cover sleep disorders in depth.

Questions readers ask

The bottom line

Morning light is the strongest everyday signal your body clock gets, and outdoor light is far brighter than indoor light, even under clouds. Get outside early, stay bright during the day, and go dim in the evening. The exact number of minutes matters less than doing it most days.

Light boxes have good trial evidence for depression, but they’re medical tools with real cautions. If you have bipolar disorder, an eye condition or take light-sensitizing medicines, check with a clinician first, and don’t swap light for treatment you already rely on.

Related reading

• Why sleep doctors oppose permanent daylight saving time
• Sleep pressure builds all day, and your body clock decides when it wins
• One in seven workers clocks in during off-hours, and sleep is the first thing to anchor
• Red-light therapy’s strongest evidence is for cancer mouth sores, not wrinkles
• Travel wellness starts with the basics, not a suitcase of supplements

Sources

1. Burns AC, Windred DP, Rutter MK, et al. “Day and Night Light Exposure Are Associated With Psychiatric Disorders: An Objective Light Study in >85,000 People.” Nature Mental Health, 2023;1(11):853–862 (Flinders University research record). researchnow.flinders.edu.au
2. Monash University. “Largest Ever Study on Light Exposure Proves Its Impact on Mental Health.” October 10, 2023. monash.edu
3. Burns AC, et al. “Day and Night Light Exposure Are Associated With Psychiatric Disorders” (preprint full text). medRxiv, 2022–2023. medrxiv.org
4. American Psychiatric Association. “Bright Light Therapy: Growing Evidence Beyond Seasonal Depression.” APA Blogs, January 6, 2025. psychiatry.org
5. Brown TM, Brainard GC, Cajochen C, et al. “Recommendations for Daytime, Evening, and Nighttime Indoor Light Exposure to Best Support Physiology, Sleep, and Wakefulness in Healthy Adults.” PLoS Biology, 2022;20(3):e3001571 (Thomas Jefferson University repository record). jdc.jefferson.edu
6. Inverse. “Light Switch” (report on the Brown et al. light-exposure recommendations, PLoS Biology). March 31, 2022, updated February 20, 2024. inverse.com
7. Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA. “A Phase Response Curve to Single Bright Light Pulses in Human Subjects.” Journal of Physiology, 2003;549(Pt 3):945–952. pmc.ncbi.nlm.nih.gov
8. Slawson DC. POEM on Menegaz de Almeida A, et al., “Bright Light Therapy for Nonseasonal Depressive Disorders: A Systematic Review and Meta-Analysis,” JAMA Psychiatry, 2025;82(1):38–46. American Family Physician, 2025;112(1):96. aafp.org
9. Mayo Clinic Staff. “Light Therapy.” Mayo Clinic health information (Mayo Clinic Care Network).
10. Mayo Clinic Staff. “Seasonal Affective Disorder Treatment: Choosing a Light Box.” Mayo Clinic Health Information Library (Mayo Clinic Care Network). amerikanhastanesi.org
11. National Heart, Lung, and Blood Institute. “Your Guide to Healthy Sleep.” NIH Publication No. 11-5271, revised August 2011. nhlbi.nih.gov
12. National Heart, Lung, and Blood Institute. “Sleep-Wake Cycle” (How Sleep Works). NIH, updated March 24, 2022. nhlbi.nih.gov
13. Riemersma-van der Lek RF, Swaab DF, Twisk J, Hol EM, Hoogendijk WJG, Van Someren EJW. “Effect of Bright Light and Melatonin on Cognitive and Noncognitive Function in Elderly Residents of Group Care Facilities: A Randomized Controlled Trial.” JAMA, 2008;299(22):2642–2655 (Vrije Universiteit Amsterdam record). research.vu.nl
14. Newswise. “Use of Bright Lighting May Improve Dementia Symptoms for Elderly Persons” (news release on Riemersma-van der Lek et al., JAMA). June 10, 2008. newswise.com