Result
Diurnal cortisol levels followed previously established rhythms
While cortisol is expected to peak in the morning and then follow a steady decrease over the day, our test cohort may have lifestyles that may impact this rhythm. Hence it became important to test if this trend was maintained. To assess the daily salivary cortisol curve, we mapped levels at the four pre-determined intervals mentioned above. We found agreement with the known 24-hour rhythmicity of cortisol concentrations as shown in Figure 1. Our results showed higher cortisol levels in the morning (3.428 ± 1.977 ng/ml), which gradually decreased through the afternoon (1.79 ± 1.55 ng/ml) and evening (1.746 ± 3.151 ng/ml), reaching their lowest point at night (0.725 ± 0.429 ng/ml). We found no dependence on gender or age for these curves as well, indicating the universal nature of this rhythm.
Figure 1: Salivary cortisol levels measured across the day, averaged across all users and days. N=93. Error bars denote S.D.
Levels of Evening Cortisol have a strong negative correlation with Sleep Score
Next, we examined the correlation between nighttime cortisol levels and the Sleep Score as well as average HRV for the sleep session (Figure 2). For this analysis, we compared the last cortisol values of the day (typically between 8-10 pm) and the app-derived Sleep Score for the following session of sleep. We observed that nighttime cortisol levels were significantly negatively correlated with sleep scores. Further, we tested a primary metric like HRV to test the strength of association with cortisol therein. With HRV we saw a weak correlation, however, this was not significant.
Figure 2: Pearson’s correlation coefficient between nighttime (8-10 p.m.) salivary cortisol levels and Ultrahuman sleep score (left), and HRV (right). N=18.
Conclusions, Limitations and Future Directions
The production, metabolism, and secretion of cortisol are tightly regulated and usually mirror the stress load in an individual 1. Therefore, elevated nighttime levels of cortisol in healthy individuals often result from chronic stress 4, circadian misalignment (such as jet lag, and shift working) 3, and even prolonged exposure to light at night 6. However, cortisol being a rapidly metabolised biochemical entity, does not lend itself to be measured on a daily basis to monitor one’s stress levels. Instead, its influence on the cardiovascular system, via the regulation of heart rate and breathing 10, are measured using proxies like photoplethysmography (PPG) to report the stress scores. However, it is important to note that cardiovascular output as measured by PPG signals can be subject to several other internal biologic processes. Hence it becomes important to test the correlation of cortisol levels with the physiologic readout being studied.
With this motivation, we assessed the cortisol levels in volunteers from the Ultrahuman team while they engaged in usual work-life activities. We found a near-typical graph of peak cortisol in the morning with a decay through the day. While peak levels were relatively lower than reference standards indicated in the test results, there is sufficient variability in reported peak cortisol values across studies for the observed values to be considered within permissible values 11,12.
Our data provided reinforcing support for the association of elevated night-time cortisol levels with adverse effects on sleep quality. Sleep score is an aggregate output of a variety of metrics including sleep duration, times awake within a sleep period, heart rate variability, and consistency in sleeping habits. Since cortisol’s hormonal effects span cardiovascular, gastrointestinal, and neuronal centers, it is not unexpected that we obtained a stronger correlation with overall sleep score rather than a single metric of HRV. Multiple other studies have similarly indicated that heightened nighttime cortisol levels are associated with reduced REM (rapid eye movement) sleep and increased periods of wakefulness during sleep
2,7. It is equally plausible that the study sample size was not large enough to parse significant associations with HRV. In a previous
retrospective analysis, we found evening heart rate based Stress Rhythm Scores were elevated for Indian Ring AIR users with evening activity, indicating a dependence on a larger sample size for this relational effect. Since this was an observational pilot study, it is our intention to follow up on the results obtained with a larger cohort.
Potential limitations of the study include the relatively small sample size and the lack of any exclusion factors employed to select the cohort, given the voluntary nature of this exercise. In participants, there may be present pre-existing conditions, lifestyle habits, chronic stress levels that can influence the outcomes obtained.
This pilot study provides preliminary evidence for the negative impact of elevated nighttime cortisol levels on sleep quality among Ultrahuman Ring AIR users. The findings underscore the importance of managing evening stress and developing effective stress management strategies for individuals whose work-life commitments expose them to regular stress closer to bedtime.
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