Evidence check: Cortisol in the context of sleep

Cortisol, the primary human glucocorticoid, follows a distinct circadian rhythm: levels rise sharply in the early morning (cortisol awakening response, CAR), peak mid-morning, decline through the day, and reach a nadir at night. This rhythm is governed by the hypothalamic-pituitary-adrenal (HPA) axis and is tightly linked to the sleep-wake cycle. Chronically elevated cortisol – often due to prolonged stress – can impair sleep quality by prolonging sleep onset, reducing deep sleep, and increasing nighttime awakenings. Conversely, sleep deprivation can activate the HPA axis, leading to elevated evening cortisol, creating a vicious cycle of poor sleep and heightened stress. The evidence for this bidirectional relationship is human-strong, supported by numerous controlled studies and meta-analyses. However, effects are highly individual; not everyone with high cortisol experiences sleep problems, and not all insomnia is cortisol-driven. A simple salivary cortisol test (morning and evening) can indicate dysregulation but does not replace clinical evaluation. Lifestyle interventions such as consistent sleep schedules, stress management, and regular exercise can help stabilize the cortisol rhythm. Commercial home tests have limited predictive value; any clinical decisions should be made by a physician. In summary, cortisol and sleep are intimately connected, but the relationship is complex and context-dependent.