The effects of season, daylight saving and time of sunrise on serum cortisol in a large population

Abstract

Cortisol is critical for maintenance of health and homeostasis and factors affecting cortisol levels are of clinical importance. There is conflicting information about the effects of season on morning cortisol and little information on the effects of sunlight on population cortisol assessment. The aim of this study was to assess whether changes in median serum cortisol occurred in a population in conjunction with changing seasons, daylight saving time (DST) or time of sunrise. We analysed serum cortisol results (n = 27,569) from a single large laboratory over a 13-year period. Subjects with confounding medications or medical conditions were excluded and data analysed in 15-minute intervals. We assessed the influence of traditional seasons, seasons determined by equinox/solstice, DST and time of sunrise on median cortisol. The median time of cortisol collection did not vary significantly between seasons. Using traditional seasons, median cortisol was lowest in summer (386 nmol/L) and spring (384 nmol/L) with higher cortisol in autumn (406 nmol/L) and winter (414 nmol/L). Median cortisol was lowest in the summer solstice quarter with significant comparative increases in the spring equinox quarter (3.1%), the autumn equinox quarter (4.5%) and the winter solstice quarter (8.6%). When cortisol was modelled against time, with adjustment for actual sunrise time on day of collection, for each hour delay in sunrise there was a 4.8% increase in median cortisol (95% CI: 3.9-5.7%). In modelling to explain the variation in cortisol over the morning, sunrise time was better than season in explaining seasonal effects. A subtle cyclic pattern in median cortisol also occurred throughout the months of the year. A 3-year trial of DST allowed comparison of cortisol in DST and non DST periods, when clock time differed by one hour. There was modest evidence of a difference in acrophase between DST and non DST cortisol (p = 0.038), with DST peak cortisol estimated to occur 58 minutes later than non-DST peak. In summary, we found that time of sunrise and time of cortisol collection were the most important factors influencing median cortisol. For each hour later that the sun rose there was an almost 5% increase in median cortisol. There was significant seasonal variability with lowest cortisol noted in summer coinciding with the earliest sunrise time. This is an important finding which is consistent with the understanding that light is the major zeitgeber in entrainment of the human circadian cortisol rhythm. Our data suggest this rhythm is resistant to the arbitrary changes in clock time with daylight saving.

The CIRCORT database: Reference ranges and seasonal changes in diurnal salivary cortisol derived from a meta-dataset comprised of 15 field studies

“…Although the presence of a circannual rhythm in cortisol levels concurs with other behavioral data (e.g., birth rates or incidence rates of psychosomatic pathologies) and ecological concepts of homeostatic change driven by environmental parameters (Foster and Roenneberg, 2008, Romero et al, 2009), many previous studies on seasonal cortisol changes documented substantial variability, with acrophases during winter (Agrimonti et al, 1982 [blood], Del Ponte et al, 1984 [blood], Hansen et al, 2001 [urine], Reinberg et al, 1978 [blood]), spring (Persson et al, 2008 [saliva]), and early summer (Hadlow et al, 2014 [blood], Matchok et al, 2007 [blood]). While part of this variability may be attributable to a sex-by-season interaction (which was nominally present, but did not reach the threshold for claiming statistical significance; see Figure 2B) and/or different characteristics of the collected specimens, a recently published Australian study (Hadlow et al, 2014) argued that the time of sunrise may be the most crucial moderator (or in this context zeitgeber ) of systematic shifts in the cortisol acrophase. In this study, a one-hour-increase in the time of sunrise was accompanied by an increase of median cortisol by approximately 5% resulting in a difference between cortisol peak and nadir of 8.6% at 30°S (which is indeed slightly below the ~10% we report for a latitude of 50°N).…”

Associations of blood pressure, sunlight, and vitamin D in community-dwelling adults

“…The reasons for the effect of season are uncertain but may relate to the fact that season strongly correlates with solar insolation and also has a strong impact on blood pressure. The mechanism(s) whereby season affect(s) blood pressure is(are) uncertain but may also relate to effects of insolation on seasonal cortisol and melatonin production [45, 46]. Further, solar insolation can vary markedly across each season, especially across different regions of the United States.…”

Seasonal Variation in Collective Mood via Twitter Content and Medical Purchases

“…From a Neuroendocrinological perspective, median cortisol levels are lowest in the summer solstice quarter and over 8.5% higher in the winter solstice quarter [4].…”