Evidence check: Cortisol in the context of Schlaf

Cortisol is a key stress hormone with a pronounced circadian rhythm: levels peak in the morning and decline throughout the day. Sleep disturbances—especially sleep deprivation, fragmented sleep, or chronic insomnia—can disrupt this rhythm, leading to elevated evening cortisol levels. This in turn impairs sleep quality and may negatively affect metabolic, immune, and mental health in the long term. The evidence for this bidirectional relationship is human-strong: numerous controlled studies and meta-analyses confirm the link between cortisol and sleep. However, a single cortisol test (e.g., saliva) is only a snapshot and insufficient for diagnosis. Repeated measurements under standardized conditions (e.g., morning/night) are necessary. Additionally, genetic factors (e.g., CRHR1 variants) can modulate stress responses, but their clinical relevance is often small. Consumer tests claiming to analyze 'cortisol and sleep' should be viewed critically: they provide hints, not diagnoses. For persistent sleep issues, medical evaluation (e.g., sleep lab, endocrinology) is recommended.