A controlled laboratory study found that older adults kept their blood sugar more stable when exposed to bright daytime light rather than constant office lighting.
Based on a monitored stay in a Dutch lab, the results suggest that indoor light schedules may play an underappreciated role in managing type 2 diabetes, alongside meals and medication routines.
For 4.5 days, volunteers 65 and older lived in rooms at Maastricht University (UM), either by broad windows or under fixed lights. Each participant returned after at least 4 weeks and switched light conditions.
The research team, led by Professor Joris Hoeks, investigates how the timing of light exposure shifts the body’s use of fuel.
Daylight guides body timing
Natural daylight varies in brightness and color, and it helps set the body’s circadian rhythm, internal 24-hour cycle guiding many functions.
“We largely spend our days under artificial lighting, which has a lower light intensity and a narrower wavelength spectrum than natural light.” noted Hoeks.
In a national survey respondents reported spending 87% of their time indoors, leaving less daylight to keep clocks aligned.
Glucose improved in daylight
Clinicians often track time in range, percent of glucose readings inside a target band, to judge day-to-day control.
One international consensus sets a common target at 70 to 180 mg/dL for most adults.
Under daylight, participants spent about 51% of the day in the trial’s normal glucose band, versus about 43% under artificial light.
Small skin sensors made continuous glucose monitoring, a wearable system that estimates glucose every few minutes, possible during both lighting sessions.
Technical glitches left incomplete sensor records for three people, so the main glucose analyses relied on ten participants.
Even so, the daylight condition produced steadier curves across the day, with smaller swings between peaks and lows.
Daylight shifted energy use
Breathing-based energy tests showed a change in fat oxidation, cells burning fat molecules for energy, during daytime hours.
Under daylight, the group relied less on carbohydrate fuel and burned more fat, especially around 1:00 PM.
Better fat use can lower the demand for insulin after meals, although the study was not designed to change medications.
Late-evening saliva tests measured melatonin, a hormone that rises at night and supports sleep timing, after each lighting period.
Levels were slightly higher under daylight during the last 2 hours before bed, from 9:00 PM to 11:00 PM.
That pattern matters because melatonin is part of the signal that night has begun for the brain and body.
How muscle clocks responded
Muscle samples let the team check whether daylight reached tissues beyond the eyes, including the skeletal muscles that clear glucose.
Lab-grown myotubes, muscle cells grown into simple fibers, showed an earlier daily clock timing after daylight exposure.
Changes in muscle timing could influence when these cells pull glucose from blood, which may help smooth daytime levels.
Daylight altered blood markers
Blood draws also fed a multi-omics, combined readout of many molecules at once, search for light-linked chemistry changes.
Compared with artificial light, daylight produced a distinct pattern in metabolites, blood fats, and immune-cell gene activity measured morning and afternoon.
Many of those differences were modest, so the team treated them as clues rather than final explanations for glucose stability.
Body clocks affect blood sugar
Circadian misalignment, sleep and meals occur at the wrong biological time, can strain glucose control even without changing food choices.
A controlled experiment found the body responded less well to insulin during misaligned schedules, even with similar sleep.
Daylight may help by strengthening daytime signals to the brain clock, which can tune fuel handling to daytime meals.
Other habits stayed controlled
Meals, sleep, stepping breaks, and screen time were tightly scheduled, so light became the main variable left to test.
That kind of standardization matters because glucose responds quickly when meal timing or movement changes, even if calories stay constant.
It also means the findings say more about lighting itself than about day-to-day choices that vary in real life.
Limitations of a small trial
Short studies can miss slow changes, and this one followed a small group of older adults for only days.
Natural light also changes with season and clouds, so results from one European setting may not map to every workplace.
Larger trials across ages and climates will be needed before daylight prescriptions become part of standard diabetes care.
Testing daylight in real life
Real workplaces include commutes, meetings, and late-night screens, so tracking light exposure in daily life becomes essential.
Researchers plan to pair light sensors with glucose monitors for several weeks, to see whether the effect persists at home.
“Could the lack of natural light be to blame for metabolic diseases such as type 2 diabetes?” asks Hoeks.
Designing buildings for daylight
Building choices can determine how much daylight reaches desks, from window size and orientation to shades and nearby structures.
Indoor lighting that mimics daylight patterns may support the main clock in the brain, which coordinates hormone release and tissue fuel use.
Design changes are not a stand-alone treatment, but they could become a low-risk layer in public health planning.
Together, the controlled trial, hormone data, tissue tests, and blood profiles all point to daylight as a metabolic signal.
Future work must test longer exposure in real buildings, while separating daylight benefits from sleep quality and activity habits.
The study is published in the journal Cell Metabolism.
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