SummaryObjectiveCortisol is a critical stress hormone with circadian rhythms synchronized by light. There are seasonal differences in expression of pro‐inflammatory genes and in some diseases moderated by glucocorticoids. As light changes with season and with latitude and longitude, we assessed changes in population cortisol associated with these parameters.DesignRetrospective data audit.PatientsPopulations across 4 states of Australia over 3 years.MeasurementsSerum cortisol levels, age, gender, time of collection, sunrise time, season and location were determined.ResultsIn 4 geographically separate populations (n = 84 937), sunrise time and time of sample collection were the most important factors influencing median cortisol. Over 2 hours in the morning cortisol could decrease by up to 76 nmol/L, and for each hour that sunrise time advanced there was up to 6.9% increase in cortisol. A cyclic seasonal pattern of cortisol was confirmed each year in all populations with autumn/winter cortisol highest compared to spring/summer with differences of up to 44 nmol/L. There was less change in cortisol in latitudes closer to the equator but cortisol progressively increased from 25 to 30°S of the equator. In more southerly latitudes, seasonal cortisol variation also increased, and over the entire latitude range, there was up to 50 nmol/L change in cortisol. Longitude variation within a time zone had a minimal effect on median cortisol.ConclusionsLocation, time of year and time of day are important influences on population cortisol levels. Elevated autumn/winter morning cortisol levels are likely due to sampling closer to the circadian peak due to later sunrise time. Understanding how the environment can influence cortisol levels may further our knowledge of physiology and disease.