Temperate country residents sleep longer and have higher sleep efficiency - regardless of the season
We compared sleep duration and efficiency in both the temperate and equatorial groups to determine how sleep profiles are affected by varying sunlight exposure. Our results show that, regardless of seasonal changes in sunlight, users in temperate countries slept longer on average than those in equatorial countries (Figure 1a). The temperate group slept on average for ~437 minutes (~7.3 hours) every night, whereas the equatorial group slept on average for ~395 minutes (~6.6 hours) every night.
Similar to sleep duration, we found that sleep efficiency was also higher in temperate countries compared to equatorial countries. Although sleep efficiency was comparable between groups for parts of the winter months (February and March), the temperate group’s sleep efficiency remained consistently higher over the study period (Figure 1b). We observed that sleep efficiency in the temperate group was ~90% on average, whereas that of the equatorial group was ~88.5% on average. We also noticed a pattern of higher variation in sleep duration and efficiency over the winter months (February-March) in both groups, which became more homogenous as temperatures rose.
Figure 1: Average a) sleep duration and b) sleep efficiency across users from January 28 to June 22, 2024, for temperate and equatorial countries. The shaded region indicates the standard error of the mean.
Recovery patterns and sleep quality are impacted differently in both groups based on the season
Sleep HRV is associated with nocturnal recovery and daily readiness; poor recovery often corresponds with decreased HRV 5. We tracked HRV across time in both latitude groups to evaluate the impact of varying sunlight exposure on recovery. Surprisingly, our findings suggest that recovery was higher in equatorial countries during the winter months compared to temperate countries - despite lower sleep duration and efficiency for the same time period. This can be explained by the consistently higher HRV in the equatorial group compared to that of the temperate group from February to March 2024 (Figure 2a). However, during the summer months, when sunlight exposure was highest, this trend reversed; the temperate group exhibited better recovery than the equatorial group. This is highlighted by the temperate group’s consistently higher HRV compared to that of the equatorial group from May to end June 2024 (Figure 2a).
RHR is another marker related to sleep quality; poor sleep quality often corresponds with elevated RHR 6. In contrast to sleep HRV, there was less variation in RHR during the winter months between the two groups. However, over time, a divergence in the curves emerged. As shown in Figure 2b, the RHR of Ring AIR users in temperate countries decreased while that of the equatorial group increased, reflecting the results of the sleep HRV shown in Figure 2a.
Figure 2: Average a) sleep HRV and b) resting heart rate across users from January 28 to June 22, 2024, for temperate and equatorial countries. The shaded region indicates the standard error of the mean.
Conclusions, Limitations and Future Directions
Human beings have been adapting to their environment for the past several thousand years. However, with the advent of modern technology, as a race, we have been able to create inventions such as indoor heating, incandescent and LED lighting, and other forms of home environment control that have profoundly lessened our exposure to climatic variations. In addition, these technologies have allowed us to remain awake and productive for a longer period of time. Hence, the seasonal variations in modern-day populations are influenced more by socio-economic drivers, through access to technology that controls home environments. The cost of this change has likely caused a modified circadian cycle that is influenced more by indoor factors than outdoor ones.
Our analysis supports this notion by revealing that the sleep volume and efficiency of residents of countries with wide ranges in sunlight remained unchanged. However, sleep recovery measured by sleep HRV and RHR, which are sensitive to temperature, indoor air quality, etc., seems to have been affected more 7. In contrast, during the winter months, residents of equatorial countries may have spent more time in tune with the external environment, which, with climbing temperatures, was negated over the study period. It is also possible that lack of exposure to sunlight is more associated with sleep recovery than sleep duration. In temperate countries, this may have been due to a genuine lack of sunlight in the January-March period, while in equatorial countries, this may be due to spending more time indoors between April and June.
This initial survey of large-scale population behaviours will inform our future investigations. We did not perform detailed statistical analysis but rather opted to profile a longitudinal description of data reporting to promote more reflection and discussion on these patterns. We did not remove any outliers or users with changes in time zones or latitudes; however, we noticed that the contribution of this subset was remarkably small.
In summary, with global changes in climate that have already started to make their impact felt,
it becomes crucial to evaluate the contributions of indoor vs. outdoor cues to an individual’s sleep profile. A study published in The Lancet investigated the link between exposure to light at night and the risk of developing type 2 diabetes
8. The researchers analysed a large cohort from the UK BioBank and found that high levels of nighttime light exposure significantly increased the risk of developing diabetes (increased glycemic dysregulation). Specifically, those in the highest exposure groups had a 53% higher risk compared to those with the lowest exposure. Such studies are critical for overall population wellness worldwide, and wearables such as the Ultrahuman AIR, coupled with
HomeHealth 9, are uniquely poised to support such data gathering and subsequent insight analytics.
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