Cortisol is a steroid hormone produced by the adrenal glands, which are small organs situated atop the kidneys. Often referred to as the body’s main stress hormone, cortisol plays a role in nearly every bodily system, affecting metabolism, immune response, and blood pressure. Its primary function is to help the body regulate its response to stress, preparing it for action during perceived threat. Beyond acute stress, cortisol also follows a predictable 24-hour cycle that is an integral part of the body’s internal clock, known as the circadian rhythm.
The Daily Cycle of Cortisol Release
The concentration of cortisol in the blood follows a distinct pattern known as the diurnal rhythm, which is characterized by a high peak in the morning and a low trough at night. This cycle is designed to align the body’s energy demands with the typical wake-sleep schedule. Levels begin to rise significantly during the second half of the night, even before a person wakes up.
The most dramatic event in this 24-hour cycle is the Cortisol Awakening Response (CAR), a rapid surge in the hormone that occurs within minutes of waking. This spike typically sees cortisol levels increase by 50% to 75% and reaches its maximum point roughly 30 to 45 minutes after a person opens their eyes.
Following this initial morning surge, cortisol concentrations decline steadily throughout the day. By the late afternoon, levels are lower, facilitating the body’s transition toward rest. The lowest point, or nadir, of cortisol production is generally reached around midnight or during the first few hours of sleep. This low level is necessary to allow for the proper onset and maintenance of restorative sleep.
The Biological Clock Governing Cortisol
The precise timing of the cortisol rhythm is governed by the body’s master biological clock, the suprachiasmatic nucleus (SCN), located in the hypothalamus region of the brain. The SCN is highly sensitive to light cues received from the retina, which act as the primary signal to synchronize the internal clock to the 24-hour day-night cycle.
The SCN regulates cortisol production through the Hypothalamic-Pituitary-Adrenal (HPA) axis, a complex hormonal cascade that links the nervous and endocrine systems. When the SCN signals the start of the active phase, the hypothalamus releases Corticotropin-Releasing Hormone (CRH). This hormone then travels to the pituitary gland, prompting it to secrete Adrenocorticotropic Hormone (ACTH) into the bloodstream.
ACTH subsequently stimulates the adrenal glands to synthesize and release cortisol. The resulting increase in circulating cortisol then creates a negative feedback loop, signaling back to the hypothalamus and pituitary to slow the release of CRH and ACTH once adequate levels are reached.
Cortisol’s Role in Waking and Energy Mobilization
The purpose of the early morning cortisol peak is to prepare the body for the metabolic and cognitive demands of the day ahead. Cortisol stimulates gluconeogenesis in the liver, which is the process of creating new glucose from non-carbohydrate sources like amino acids and fats.
This mobilization of stored energy ensures that the brain and muscles have the necessary fuel to engage in activity after the overnight fast. Furthermore, cortisol acts on adipose tissue to promote lipolysis, releasing fatty acids that other cells can use for energy.
Beyond energy, the morning surge also contributes to the heightened alertness and cognitive function required for wakefulness. The increase in cortisol sharpens mental focus and is believed to play a role in consolidating memory and regulating mood.
Factors That Disrupt the Natural Cortisol Rhythm
The predictable 24-hour cortisol rhythm is susceptible to alteration by various external and lifestyle factors, which can change the timing or magnitude of the hormone’s release. Chronic psychological or physiological stress is a primary disruptor, leading to sustained activation of the HPA axis and potentially resulting in consistently elevated cortisol levels throughout the day. This prolonged activation can flatten the diurnal curve, preventing the necessary evening decline.
Inconsistent sleep schedules, such as those experienced by shift workers or due to frequent jet lag, directly conflict with the SCN’s programming. When the sleep-wake cycle is erratic, the body’s clock struggles to accurately time the cortisol peak, often resulting in a blunted or delayed Cortisol Awakening Response.
Exposure to artificial light at night interferes with the SCN’s function. Light signals the brain that it is daytime, which can suppress the nighttime production of melatonin and prematurely stimulate the HPA axis. Consequently, this inappropriate signaling can shift the timing of the cortisol trough and peak, disturbing the natural rhythm and hindering the quality of restorative sleep.