As summer winds down, it’s a great time to reflect on the benefits of daylight exposure. This article looks at time in daylight, as estimated by Apple Watch, across Apple Heart and Movement Study participants in 2024.
In our modern world, where many spend hours indoors, it’s easy to overlook how crucial natural daylight is for our overall well-being. From mental health and mood to eyesight and vitamin D levels, daylight plays a vital role in keeping us healthy.1-8 Let’s explore how daylight exposure is measured, the trends we’re seeing in participants who provided informed consent to join the Apple Heart and Movement Study and elected to share this type of information with researchers, and the known benefits and potential drawbacks of sun exposure.
What is Time in Daylight?
Traditionally, in research, sun exposure has been estimated with survey questions directly asking about the amount of time spent outside, using ultraviolet radiation dosimeters or approximated from the blood levels of Vitamin D, and from a population standpoint, based on latitude, where UV strength and daylight availability can vary greatly by season.3,6,7
In the Fall of 2023, Apple introduced a new metric called “Time in Daylight” as part of watchOS 10 with Apple Watch Series SE (second generation) and Apple Watch Series 6 or later. This metric uses the ambient light sensor in Apple Watch and an associated algorithm to estimate how much time is spent in daylight and maximum intensity (lux) in 5-minute increments. Users can review their estimated time in daylight in the Health app, and also see its association with their moods and emotions in the State of Mind section. Participants in the Apple Heart and Movement Study had the option to share this data with the Study, giving us a new way to explore daily sun exposure and its impact on health.
Time in Daylight amongst Apple Heart and Movement Study Participants
To be included in this analysis, participants needed to wear their watch for at least eight hours between 5:00 am and 10:00 pm and record at least one minute of daylight in any given month. If a participant recorded zero minutes of daylight on a given day, that day was still included in their average. From January 1st to July 31st, 2024, there were 58,350 participants that met these criteria, contributing collectively, a total of over 9.6 million days of daylight estimates (see Table 1 for demographic characteristics).
As shown in Figure 1, the average time spent in daylight increased dramatically from an average of 30.8 minutes in January (95% CI 29.0–32.5 min) to a peak of 102.1 minutes in June (95% CI 97.1–107.1 min) across the entire country. It is unsurprising that the highest average is in June when the US experiences the longest days of the year from mid-June to mid-July.
Figure 1: Mean and 95% CI of the average estimated daylight exposure per participant by month. Participants’ averages were considered for each month if at least 1 minute of daylight was recorded. See Table 2 for total participants, monthly average, and 95% CI values.
Looking at daily averages (Figure 2), weekends consistently show higher daylight exposure. Of the top ten days of 2024 so far, the 4th of July – which fell on a Thursday this year – had the highest average daylight exposure of 138.0 minutes. The other top days for sunlight exposure were all Saturdays or Sundays. While June 20th was the longest day of the year, we see that day of the week, holidays, and weather patterns across the US have the ability to heavily influence these daily averages.
Figure 2: Participants’ daily average estimated minutes in daylight. Participants increase sun exposure from January peaking in June to mid-July. January 9th had the least estimated minutes (20.6 min), and the 4th of July had the highest average (138.0 min).
How do seasons affect daylight exposure by state?
Seasonal changes impact daylight exposure at the state level. Figure 3 illustrates state-level averages for January, April, and July. While most states show an upward trend as days get longer, a few exceptions stand out. Hawaii, for example, remains fairly consistent across all three months, leading the nation in January and April, and ranking fifth in July. Interestingly, some southern states’ averages declined from April to July, with Louisiana (-5.5 min) and Florida (-5.3 min) seeing the largest drops, along with Mississippi, Georgia, and Arizona. This could likely be due to residents avoiding the extreme summer heat during daylight hours.
Figure 3: The unadjusted average number of minutes participants spent in daylight by state residence in a.) January (winter), b.) April (spring) and c.) July (summer) of 2024. The same scale is used in all three maps. The data presented in this figure is viewable in Table 3, including details by state and the possible minutes of daylight in each state estimated based on that state’s capital.
Who gets the most sunlight and what associations do we see related to mental health?
When comparing average daylight exposure with demographic information, we found that older adults (up to age 74) tend to get more sun, and men generally have higher exposure than women (see Figure 4). Across the cohort, Figure 4 shows that as age increases up to 74 years, there is an increase in minutes. We also saw trends of association between time in daylight with employment status (retired as highest, unable to work as lowest), perceived socioeconomic status, exercise minutes recorded on Apple Watch, and various medical conditions.
Figure 4: This bar chart demonstrates the average daylight minutes estimated per day from Jan 1st – July 31st, 2024, grouped by age and sex. The data presented in this figure is viewable in Table 4.
One question we were interested in exploring is the relationship between daylight exposure and stress levels, as measured by the Perceived Stress Scale 4 (PSS-4).10 Participants are presented with this survey monthly, and the resultant scores range from 0 to 16, with higher scores correlating with higher stress levels. We considered a score greater than 6 to be high stress based on prior literature and the distribution of PSS-4 scores among this sub-cohort.11
To account for known differences in the population and possible confounders; a logistic regression analysis was done to assess the contributions of different factors to a PSS score higher than 6. To be considered for this analysis, we required participants to have recorded exercise minutes on at least one day in May 2024, to have filled out a medical history survey at least once, and to complete a PSS-4 survey between May 15th and June 14th, 2024, for a cohort size of N = 16,509. The average daily minutes of sunlight exposure were calculated for each person using daylight data collected in May 2024-when state-specific differences are at their lowest. Daylight exposure was categorized based on the cohort’s rounded quartiles of less than 40 minutes (1st quartile), between 40 – 60 minutes (2nd quartile), 60 – 90 minutes (3rd quartile), and more than 90 minutes (4th quartile). In addition to the average minutes of daylight exposure in a day, the model adjusted for factors including age, assigned sex, ethnicity, self-reported relative socioeconomic status, employment status, BMI, average daily exercise minutes recorded on Apple Watch, smoking status, and medical conditions including having cardiovascular conditions, breathing disorder (i.e., sleep apnea or asthma), hypertension, diabetes, anxiety, and depression.
In this model, the odds of a PSS-4 score above the threshold associated with high perceived stress were 22.8% (95% CI [14.3%, 30.5%], p-value = 0.0000) lower among those with the longest sunlight exposure (i.e., more than 90 minutes in a day) than those with the shortest sunlight (i.e., less than 40 minutes in a day), see Table 5 for more details. This correlation between sunlight exposure and lower stress has been reported in the past, and, thanks to the engagement of study participants, the Apple Heart and Movement Study has the potential to help understand these relationships better.11
Known benefits of the sun
Beyond brightening our days, sunlight plays a vital role in various aspects of our health and well-being, offering numerous benefits. Some of these include:
- Daylight helps maintain healthy eye function and can reduce the risk of nearsightedness in children and adolescents1,2
- Vitamin D, synthesized through sunlight exposure, is crucial for calcium absorption in the body, which in turn is vital for maintaining strong and healthy bones. Adequate Vitamin D levels help prevent bone disorders such as osteoporosis and rickets.3
- Sunlight and light therapy are used in the treatment of psychological disorders such as seasonal affective disorder (SAD), depression, and bipolar disorder, and are important factors in the regulation of serotonin and melatonin regulation.4-7
- Exposure to natural light during the day, especially in the morning, helps regulate your circadian rhythm.8
Tips for Increasing Daylight Exposure
While everyone has different home and work situations, it’s important to get outside year-round, even when it’s cold. Here are a few ways to increase those minutes spent in the sun:
- Morning Walks: Start your day with a walk outside to get early sunlight. If walking isn’t in the cards, sit outside with your first cup of tea or coffee.
- Work Near Windows: Position your workspace near a window to benefit from natural light.
- Outdoor Activities: Engage in outdoor activities like gardening, hiking, or sports and try out new hobbies (birdwatching or pickleball, anyone?) that allow more time outside.
- Breaks Outside: Take regular breaks outside during work or study sessions.
It’s important to remember that while moderate sun exposure is beneficial, too much sun can be harmful. Overexposure to UV rays can cause skin damage, with premature aging of skin and an increased risk of skin cancer. It’s essential to balance sun exposure with skin protection, by avoiding direct sunlight during peak hours and by using sunscreen when appropriate. The Apple Heart and Movement Study is an observational study and not intended to provide medical advice. Participants are encouraged to discuss sun exposure with their health care providers.
Conclusion
The insights from the Apple Heart and Movement Study highlight how daily sun exposure varies throughout the year and across different demographics along with the association we see between sunlight exposure and stress. By making small changes to increase your time in daylight, you may be able to enjoy the many health benefits it offers.
As always, thank you to all our AHMS participants for contributing your data and time to help pursue new questions relating to Time in Daylight and its impact on health, mood, and beyond.
Viewing Time in Daylight
- Open the Health app on your iPhone
- Tap browse at the bottom right, then tap Other Data (or Mental Wellbeing)
- Tap time in daylight
- To adjust the view of your data across time, the top bar allows you navigate between day (D), week (W), month (M), 6 months (6M), and year (Y).
For more information, please see:
https://support.apple.com/guide/watch/see-time-in-daylight-apd3ab22534c/watchos
Get the app. Join the Study.
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2. Lingham, G., Yazar, S., Lucas, R.M. et al. Time spent outdoors in childhood is associated with reduced risk of myopia as an adult. Sci Rep 11, 6337 (2021). https://doi.org/10.1038/s41598-021-85825-y
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7. Yiwen Cao, Jipeng Zhang and Rui Feng*. Negative Association Between Sun Exposure and Depression: A Cross-Sectional Analysis of the NHANES 2011-2018 In the United States. Am J Biomed Sci & Res. 2023 – 20(4). AJBSR.MS.ID.002726. DOI:
8. Blume C, Garbazza C, Spitschan M. Effects of light on human circadian rhythms, sleep and mood. Somnologie (Berl). 2019 Sep;23(3):147-156. doi: 10.1007/s11818-019-00215-x. Epub 2019 Aug 20. PMID: 31534436; PMCID: PMC6751071.
9. Wang J, Jeevarathinam AS, Jhunjhunwala A, Ren H, Lemaster J, Luo Y, Fenning DP, Fullerton EE, Jokerst JV. A Wearable Colorimetric Dosimeter to Monitor Sunlight Exposure. Adv Mater Technol. 2018 Jun;3(6):1800037. doi: 10.1002/admt.201800037. Epub 2018 Apr 23. PMID: 33928184; PMCID: PMC8081380.
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11. Malik AO, Peri-Okonny P, Gosch K, et al. Association of Perceived Stress Levels With Long-term Mortality in Patients With Peripheral Artery Disease. JAMA Netw Open. 2020;3(6):e208741. doi:10.1001/jamanetworkopen.2020.8741
First photo featured: iStock.com/SStajic