A new study published in the Journal of Health Psychology has found that exposure to sunlight in the morning may improve sleep quality later that night. Individuals who spent time in the sun during the morning reported better sleep quality, regardless of how much total sunlight they received throughout the day. The results suggest that morning sunlight may help regulate circadian rhythms.
Sleep is increasingly recognized as critical to health, influencing outcomes such as obesity, diabetes, hypertension, depression, and general well-being. However, sleep duration appears to be declining in the general population, with more people reporting insufficient rest. Previous research has explored various factors that can influence sleep, including diet, exercise, and socio-economic conditions.
Sunlight, a key regulator of circadian rhythms and melatonin production, has been linked to sleep in prior studies. However, there has been limited research on how daily sun exposure, especially its timing, affects sleep. The researchers aimed to fill this gap by investigating how morning, mid-day, and evening sunlight exposure correlates with sleep quality.
The study followed 103 adults over a 70-day period. Participants were recruited from an online pool and ranged in age from 18 to 80, with roughly equal representation across gender and age groups. Each participant filled out a daily survey about their sunlight exposure and sleep quality, which they completed between 8:30 PM and midnight local time.
Participants reported how long they spent outside in direct sunlight and what part of the day they were outside—morning, mid-day, or evening. Sleep quality was measured using a modified version of the Pittsburgh Sleep Quality Index, a widely used tool that assesses various aspects of sleep, such as total sleep time, ease of falling asleep, and subjective sleep quality.
To control for factors that could influence sleep, such as age, health, and whether the day was a weekend or weekday, the researchers used multilevel modeling. This statistical method allowed them to differentiate between long-term patterns (how individual sleep patterns change over time) and daily fluctuations (how sunlight exposure one day affects sleep that night).
The study’s most significant finding was that morning sunlight exposure predicted better sleep quality that night. People who spent time outside in the morning reported falling asleep more quickly, sleeping longer, and experiencing fewer awakenings during the night. Their sleep efficiency—how much of the time they spent in bed was spent asleep—was also higher. These findings held even when controlling for other variables, such as the quality of their previous night’s sleep.
Interestingly, the total amount of time participants spent in sunlight each day did not have a significant impact on sleep quality. This suggests that it’s the timing of sunlight exposure, not the overall duration, that plays a critical role in promoting better sleep. Morning sunlight, in particular, may help synchronize circadian rhythms—the body’s internal clock that regulates sleep-wake cycles—leading to improved sleep quality.
There were no consistent associations between mid-day or evening sunlight exposure and sleep quality. However, some secondary analyses found that individuals who spent time in evening sunlight tended to wake up earlier the following morning. But overall, it was morning sunlight that consistently predicted better sleep.
As with any research, this study has its limitations. One notable constraint is that participants self-reported their sunlight exposure and sleep quality, which could introduce bias. Additionally, the study didn’t account for certain variables that might influence sleep, such as the intensity of sunlight, weather conditions, or the participants’ exposure to artificial light. For instance, someone who lives in a northern climate with shorter days in winter might have different results compared to someone living in a tropical climate with consistent daylight year-round.
The findings from this study open the door to several avenues for future research. First, it would be valuable to investigate whether the benefits of morning sunlight exposure differ by geographical location, skin pigmentation, or season. For example, people living in regions with long winters and limited daylight might have different sunlight exposure patterns compared to those living closer to the equator. Additionally, future studies could examine how artificial light—such as from screens or indoor lighting—interacts with natural sunlight to affect sleep.
The study, “Does sunlight exposure predict next-night sleep? A daily diary study among U.S. adults,” was authored by Austen R Anderson, Lindsey Ostermiller, Mallory Lastrapes, and Lauren Hales.