Morning sunlight shifts sleep cycles earlier and boosts quality

Spending more time in the sun early in the morning may help people fall into healthier sleep patterns, according to a new study published in BMC Public Health. Researchers found that morning light exposure shifts sleep timing earlier and improves sleep quality.

Scientists have long known that sunlight plays a crucial role in regulating the body’s internal clock, which helps determine when people feel alert and when they feel sleepy. This internal clock relies heavily on light signals from the environment, particularly natural daylight. In recent decades, however, many people have spent less time outdoors due to office work, screen use, and urban living. These trends intensified during pandemic lockdowns, when outdoor movement was limited for months at a time.

Led by Luiz Antônio Alves de Menezes-Júnior from the Federal University of Ouro Preto in Brazil, the researchers behind the new study wanted to better understand whether the timing of sunlight exposure matters, not just the total amount of sunlight people receive. Previous research suggested morning light might be especially important, but few large population studies had tested this idea in real-world settings.

To explore this question, the scientists surveyed 1,762 adults living in two Brazilian cities between October and December 2020. Participants reported how often and how long they were exposed to sunlight at different times of day—before 10 a.m., between 10 a.m. and 3 p.m., and after 3 p.m. They also answered detailed questions about their sleep habits, including how long they slept, how quickly they fell asleep, and when they went to bed and woke up.

One key measure examined in the study was the “midpoint of sleep,” which represents the halfway point between falling asleep and waking up. This measure is important because it reflects how well a person’s sleep schedule aligns with their internal body clock.

The findings showed that morning sunlight had the strongest influence on sleep timing. For every additional 30 minutes of sunlight exposure before 10 a.m., the midpoint of sleep shifted earlier by about 23 minutes. In practical terms, this means individuals who spent more time in morning sunlight tended to fall asleep and wake up earlier, aligning their sleep more closely with natural day-night cycles.

Sunlight exposure after 3 p.m. also shifted sleep timing earlier, but the effect was smaller. Midday sunlight showed no clear link to sleep timing. Importantly, the study also found that more morning sunlight was associated with better overall sleep quality, while total sleep time and time spent falling asleep were largely unaffected.

The researchers believe morning sunlight helps reset the body’s internal clock, sending a strong signal that it is time to be awake and alert. This signal then helps the body prepare for sleep later that evening. Without enough early-day light, the body clock can drift later, leading to delayed sleep and difficulty waking up.

“Morning sunlight, in particular, helps regulate the secretion of melatonin, a hormone crucial for sleep regulation, thereby improving sleep onset and sleep quality. Increased sunlight exposure also correlates with lower levels of daytime sleepiness and improved alertness during the day,” the authors explained.

The study does have limitations. For instance, it did not control for other exposure to artificial light, such as screens, which also impacts the body clock. Additionally, it relied on self-reported data; thus, the results may be affected by memory errors or personal bias.

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.