From the Research
The optimal time to check cortisol levels is in the early morning, between 7 AM and 9 AM, when cortisol naturally reaches its peak in the body. For most accurate results, the blood sample should be collected after fasting overnight and before taking any medications that might affect cortisol levels 1. If you're having a 24-hour urinary free cortisol test, you'll need to collect all urine over a full 24-hour period. For salivary cortisol testing, samples are typically collected at specific times throughout the day (morning, noon, evening, and midnight) to assess your cortisol rhythm.
Some key points to consider when checking cortisol levels include:
- Avoiding strenuous exercise, smoking, and consuming alcohol before the test, as these can affect results 2
- Considering the diurnal pattern of cortisol, with highest levels in the morning and lowest around midnight, which helps regulate metabolism, immune function, and stress response throughout the day 3
- Being aware that light exposure in the early morning can increase cortisol levels, with a study showing a 35% increase in salivary cortisol levels after awakening when exposed to light of 800 lux 4
- Understanding that individual differences in morningness-eveningness can affect the circadian rhythm of cortisol and body temperature, with "larks" (morning chronotypes) having an earlier peak time than "owls" (evening chronotypes) 5
It's essential to follow the specific guidelines and instructions provided by your healthcare provider for the test, as they may vary depending on the type of test and individual circumstances. The most recent and highest quality study 2 recommends a new serum cortisol cutoff of 14 to 15 μg/dL, depending on the assay used, for the diagnosis of adrenal insufficiency, highlighting the importance of using up-to-date and accurate reference values when interpreting cortisol levels.