Cortisol and Circadian rhythm
Cortisol levels vary throughout the day and follow a circadian (24-hour) rhythm. Cortisol levels begin to increase significantly in the early morning, peaking around mid-morning. Their levels then gradually decline throughout the day until they reach their lowest levels during nighttime sleep. Approximately 77% of people also experience the cortisol awakening response (CAR), which is a sharp increase in cortisol 30min after waking that is independent of the normal circadian rhythm.
Cortisol and Stress
Stress is anything that strains homeostasis, which is the capacity of the body to maintain its biological processes such as temperature or heart rate. Stress can be external factors such as work and social stressors or internal such as dysregulation of vital processes. In response to these factors the body typically engages a stress response that serves to return it to a state of normalcy. This response involves increased cognition, lower sensitivity to painful stimuli, an increase in blood sugar levels, increased breakdown of fats and other lipids and an inhibition of reproduction.