More light during the day means better sleep at night: what scientists have discovered

Researchers observed 89 adults in their everyday lives and found that people who spent more time in bright daylight tended to fall asleep earlier and wake up earlier. A more consistent lighting pattern was associated with more pronounced deep sleep during the first half of the night.

The study does not prove that a walk in the sunshine will cure insomnia. It shows a link between daytime light exposure and sleep patterns. Furthermore, sleep stages were assessed using a consumer fitness tracker rather than laboratory equipment.

How light regulates sleep patterns

The body has an internal biological clock. This helps determine when a person feels alert, when they start to feel tired, and at what time it is easier to fall asleep.

Light serves as the main external cue for this clock. Bright light in the morning tells the brain that it is daytime. In the evening, the reduction in light signals that it is time to prepare for sleep.

The problem with modern lifestyles is that this distinction is gradually becoming blurred. During the day, many people hardly ever leave their homes and are exposed to significantly less light than they would be outdoors. Yet in the evening, they continue to sit with the lights on and stare at bright screens.

As a result, the body may receive a confusing signal: it is not light enough during the day, and not dark enough at night.

What exactly did the researchers measure?

The study involved 89 adults. Over the course of a week, they wore a light sensor and a consumer sleep tracker, and also recorded their sleep patterns daily.

In total, the researchers collected data covering more than 500 days.

The sensor measured not just visible brightness, but what is known as melanopic illuminance. This is an indicator of the extent to which light affects specific cells in the retina that are involved in regulating the body’s biological clock.

Put simply, this sensor assessed not only how bright a person perceives their surroundings to be, but also how strong a ‘daytime signal’ their body is receiving.

What the results showed

People who spent longer periods in bright light during the day fell asleep earlier and woke up earlier, on average.

The consistency of the routine was also significant. A more stable distribution of light throughout the week — without sudden and erratic shifts between dim and bright lighting — was associated with healthier sleep patterns.

Participants with the most regular light patterns also had more deep sleep at the start of the night, as measured by a wearable tracker.

Deep sleep is particularly pronounced in the first few hours after falling asleep. It is associated with the body’s recovery, the functioning of the immune system, information processing and the consolidation of memories.

Why the body needs a bright day

Light helps the body’s internal clock distinguish between day and night. The clearer this contrast, the easier it is for the body to maintain a stable rhythm.

If a person spends the whole day in a dimly lit room, their biological clock may receive only a weak signal that the day has begun. Artificial lighting in the evening further delays the body’s natural preparation for sleep.

Brighter daylight, on the other hand, can strengthen the circadian rhythm. The body has a better understanding of when to stay alert and when to begin the transition to night-time rest.

This does not mean that one needs to be constantly exposed to dazzlingly bright lights during the day. The study shows a general correlation: a more pronounced and regular daylight signal was associated with more regular sleep patterns.

Why regularity is important

Scientists found that it was not only the overall brightness that mattered, but also the predictability of the lighting.

If a person goes outside early one day, sees hardly any daylight the next, and spends a brightly lit evening well into the night on the third, it is harder for their internal clock to maintain a stable rhythm.

Regular lighting works much like a timetable. A period of brightness is repeated during the day, and a period of darkness in the evening and at night. This allows the body to regulate sleep, wakefulness and other processes dependent on the time of day more precisely.

According to the study, a less chaotic light pattern was associated with more pronounced deep sleep during the first part of the night.

So, does this simply mean we should go outside more often?

The study supports a simple general principle: during the day, it is beneficial to get enough light – preferably natural light – and in the evening, to gradually reduce the brightness of the lighting.

However, the authors did not identify a universal ‘dose’ of light. The study does not specify exactly how many minutes one should spend outdoors, at what time this is best done, or what level of brightness is required for each individual.

Nor should the findings be interpreted as a treatment for insomnia. Sleep disturbances can be linked to stress, pain, medication, sleep apnoea, anxiety disorders, work patterns and a host of other causes.

Daylight is one factor in a person’s daily routine, not a universal remedy for all sleep problems.

What changes can be made to everyday life

Several cautious practical conclusions can be drawn from the study.

In the morning and during the day, it is worth opening the curtains whenever possible, working closer to a window and going outside. Even on a cloudy day, natural daylight is usually noticeably brighter than indoor lighting.

In the evening, it is helpful to reduce the overall brightness of the light, especially before going to bed. It is not just your smartphone screen that matters, but all the surrounding lighting too — ceiling lights, the television and bright lamps.

Consistency is just as important: waking up, being active during the day and going to sleep at roughly the same times each day helps the body to better distinguish between the light and dark parts of the day.

However, these are general principles of sleep hygiene, not individual medical advice.

People do not always accurately assess their own sleep

Participants also recorded their own impressions of their night’s rest. Overall, their assessments matched the data from the wearable devices.

However, where sleep patterns were more disrupted, the discrepancy increased. When the tracker showed less deep or rapid eye movement (REM) sleep, a person might assess the previous night differently from the device.

This shows that the subjective experience of sleep and its measurable characteristics do not always coincide. It is possible to sleep for a sufficiently long time but still feel exhausted — or, conversely, to consider the night a poor one, even though the key indicators remained relatively stable.

Why this matters

Sleep is usually discussed in terms of evening habits: avoiding coffee, mobile phones and working late. New research shifts some of the focus to daytime hours.

The body needs not only a dark night, but also a sufficiently bright day. If the overall light exposure becomes dim, irregular and uniform around the clock, the body’s internal clock finds it harder to maintain a clear schedule.

This is particularly relevant for people who work from home, rarely go outside, live in regions with short winter days, or work rotating shifts.

Background

Laboratory studies have long shown that light can shift the times at which we fall asleep and wake up, suppress melatonin production and influence circadian rhythms.

This new study is significant in that it examined real-life conditions rather than artificially created laboratory settings. The researchers demonstrated that readily available sensors can track the link between actual lighting conditions and sleep over the course of several days.

That said, the study does not identify an ideal routine for everyone. Age, individual biological clocks, the season and lifestyle can significantly alter a person’s response to light.

Source

Altug Didikoglu et al.,“Light Exposure and Sleep Architecture in Real-World Settings”, npj Biological Timing and Sleep, 2026.