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Exposure to natural sunlight early in the day is associated with earlier sleep schedules and better overall rest. Researchers found that adults who spent more time in the morning sun had a more aligned internal biological clock, which can promote healthier sleep patterns. The study was published in the journal BMC Public Health.
The human body relies on an internal biological clock, known as the circadian rhythm, to manage sleep and wake cycles. This rhythm is governed by a tiny region in the brain called the suprachiasmatic nucleus. When light enters the eyes, signals travel to this brain region, which then guides the release of various chemical messengers.
One major messenger is melatonin, a hormone that promotes sleepiness. When darkness falls, the body produces more melatonin. When the eyes detect bright light, melatonin production drops, promoting wakefulness.
Modern lifestyles limit how much natural light people receive. Many adults work indoors or spend their leisure time looking at screens. During the COVID-19 pandemic, lockdowns and remote work dramatically reduced the amount of time people spent outside.
Extended periods spent indoors can disrupt the biological clock. When a person’s biological time falls out of sync with their daily obligations, such as waking up early for work despite not feeling rested, they can experience chronic exhaustion. This misalignment is sometimes called social jetlag.
Lead researcher Luiz Antônio Alves de Menezes-Júnior, a public health and nutrition researcher at the Federal University of Ouro Preto in Brazil, and his colleagues wanted to understand how sunlight at different times of the day affects specific elements of sleep. The researchers focused heavily on a metric called the midpoint of sleep, which is the exact halfway mark between the time a person falls asleep and the time they wake up. An earlier midpoint indicates a sleep cycle that is well-synchronized with the natural progression of day and night. A delayed midpoint can mean the internal clock is out of sync with typical social and work schedules.
The researchers conducted a cross-sectional survey of 1,762 adults living in the Iron Quadrilateral region of Brazil between October and December 2020. Interviewers visited homes in two mid-sized cities, using protective measures to prevent the spread of the coronavirus. They randomly selected households from census data, aiming to capture a representative sample across various socioeconomic groups. The participants answered detailed questions about their general health, socioeconomic status, and daily habits.
To evaluate sleep, the participants completed the Pittsburgh Sleep Quality Index, which is a validated questionnaire used to measure different dimensions of rest over the previous month. This tool helped the researchers calculate each person’s total sleep time, the amount of time it took them to fall asleep, and their sleep efficiency. Sleep efficiency is the percentage of time a person spends actually sleeping while in bed. The questionnaire also provided the data needed to calculate each participant’s midpoint of sleep and their overall sleep quality score.
Overall, the participants reported an average total sleep time of just over seven hours per night. The average sleep midpoint was around 5:18 a.m., meaning the typical participant fell asleep shortly before midnight and woke up just after 7:00 a.m.
To track light exposure, the researchers asked the participants to estimate how many days a week they spent time in the sun. The participants then estimated how many minutes they spent in the sun on those days across three specific time windows. These windows were before 10 a.m., between 10 a.m. and 3 p.m., and after 3 p.m.
The research team used statistical models to look for relationships between the timing of sunlight exposure and the various sleep metrics. To analyze the results, they used a directed acyclic graph, which is a theoretical model that maps out how different variables might influence one another. This helped them select the correct factors to adjust for in their models, avoiding false associations. They factored in variables such as age, gender, education level, chronic health conditions, and levels of physical activity.
The researchers found that morning sunlight exposure had the strongest relationship with the midpoint of sleep. For every additional 30 minutes a participant spent in the sun before 10 a.m., their sleep midpoint shifted earlier by an average of 23 minutes. The findings suggest that morning light serves as a primary signal to help reset the biological clock.
Afternoon sunlight exposure also showed a relationship with the sleep midpoint, though the effect was smaller. Every 30 minutes of sun exposure after 3 p.m. was associated with a 19-minute earlier sleep midpoint. The researchers noted that afternoon sunlight might signal the approaching evening, or it might coincide with outdoor physical activity that generally aids sleep.
Sunlight exposure in the middle of the day, between 10 a.m. and 3 p.m., had no statistically significant association with the midpoint of sleep. The lack of an association between midday light and sleep timing suggests that the biological clock is most responsive to light at the extremes of the day.
Beyond the sleep midpoint, morning sunlight was the only time window associated with a better overall sleep quality score on the questionnaire. People who spent more time outside before 10 a.m. reported generally better rest.
Conversely, sunlight exposure at any time of day showed no statistically significant association with total sleep time. Light exposure also had no statistically significant association with how quickly participants fell asleep or their sleep efficiency.
The study relies on observational data, meaning it can only identify patterns rather than prove that sunlight exposure directly causes sleep changes. The participants estimated their own sun exposure and sleep habits, which can introduce memory errors or biases compared to using objective tracking devices.
The researchers calculated a single average sleep midpoint for each participant for the entire week. They did not separate sleep data for workdays versus weekends. People often sleep in on their days off to catch up on rest. A broader weekly average might mask the specific effects of varying daily schedules and weekend sleep habits.
The analysis did not track exposure to artificial light or the amount of time participants spent looking at screens in the evening. Late-day screen light is known to influence sleep onset and biological rhythms, which could skew the interpreted effects of natural sunlight. Additionally, the sleep efficiency data was based entirely on self-reports. Measuring true sleep efficiency generally requires clinical tools like movement-tracking wristbands or formal sleep studies in a laboratory setting.
The data collection took place during the first year of the COVID-19 pandemic, a period marked by high social isolation, anxiety, and altered work routines. These unique societal conditions likely influenced the participants’ stress levels and sleep patterns in ways that are difficult to separate from their light exposure. Future research could use wearable technology to measure outdoor light exposure and sleep patterns precisely, helping clarify exactly how natural light guides the human biological clock under normal daily conditions.
The study, “The role of sunlight in sleep regulation: analysis of morning, evening and late exposure,” was authored by Luiz Antônio Alves de Menezes-Júnior, Thais da Silva Sabião, Júlia Cristina Cardoso Carraro, George Luiz Lins Machado-Coelho, and Adriana Lúcia Meireles.