It’s a hot topic in sleep research: the relationship between cortisol and the quality and patterns of sleep. I’ve been talking about cortisol for a while, but I’ve never devoted a standalone article to this important topic. It’s time to correct that.
Today, I’ll talk about the role that cortisol plays in the sleep-wake cycle, how disruptions to healthy cortisol levels interfere with sleep and contribute to sleep disorders—and how poor sleep, in turn, negatively affects cortisol. I’ll also discuss ways to encourage healthy cortisol levels, for the benefit of your sleep and broader health.
What does cortisol do?
Cortisol is a stimulating, alerting hormone. It’s the body’s primary stress hormone—that’s the role that gets cortisol most of its attention. Urged on by a complex network that incorporates elements of the central nervous system and the adrenal system, cortisol drives the body’s fight-or-flight response in the presence of a threat or stressor.
But cortisol does more than spur fight-or-flight. This hormone has a number of other functions, including:
- Regulating blood pressure
- Balancing blood sugar
- Influencing inflammation
- Regulating energy levels
- Contributing to the cardiac system function
- Helping to control the sleep-wake cycle
Cortisol gets a pretty bad rap these days—and there’s no question that chronically elevated cortisol contributes to sleep disruptions and other health problems (more on those in a moment). But it’s important to be clear: Cortisol is an essential component of human physiology. The challenge for many of us is to keep cortisol levels from veering too high. (As you’ll see, sleep can help with that.)
When cortisol is elevated too frequently and over long periods of time, it can cause a number of health problems. They include:
- Chronic illnesses, including high blood pressure, diabetes, and heart disease
- Weight gain (both by stimulating appetite and by encouraging the body to store fat more aggressively)
- “Foggy brain,” and difficulty with memory and focus
- Compromises to the immune system, increased inflammation, and greater vulnerability to illness, disease, and other effects of aging
- Problems with digestion
- Mood disorders, including depression and anxiety
- Sleep problems
Cortisol doesn’t operate in isolation. It’s part of a complex system known as the HPA axis (that’s short for the hypothalamic-pituitary-adrenal axis), which combines parts of the central nervous and endocrine systems. Cortisol is produced in the adrenal glands, and the hypothalamus and pituitary gland, located in the brain, monitor cortisol levels and send messages to the adrenal system to adjust its production, depending on the body’s needs and circumstances. It’s the complex, dynamic communication of the HPA axis that produces cortisol and helps to regulate body functions ranging from sleep-wake cycles to stress and mood to digestion and immune function.
Cortisol is a major—but not the only—hormone that functions within this system, with direct effects on sleep. The sleep-facilitating hormone melatonin is another. Together, melatonin and cortisol work within the HPA axis to regulate sleep and wakefulness.
When it comes under prolonged or chronic stress, this network can become constantly activated, the hypothalamus and pituitary gland constantly signaling the adrenal system to produce more cortisol. It is cortisol’s role as part of this axis that’s attracted a lot of attention from sleep scientists in recent years. That’s because chronic stress is such a widespread problem with such deep effects on sleep. It’s also because cortisol and the HPA axis it operates within interact with sleep in several different and important ways.
What to remember: Cortisol is more than a stress hormone—it also plays a major role in regulating sleep and other important physiological functions, all from within a network known as the HPA axis.
The cortisol rhythm and sleep
Like nearly all hormones in the human body, cortisol has a daily, 24-hour rhythm. For most biotypes, cortisol levels are at their highest in the morning, usually around 9 a.m. Cortisol begins to rise gradually in the second half of a night’s sleep. The hormone begins a more rapid rise around the time you’re waking up before peaking at about 9. From that point on, cortisol makes a gradual decline throughout the day, reaching its lowest levels around midnight. The activity of the HPA axis, which produces cortisol, reduces to its lowest levels in the evenings, right around your bedtime. In this way, cortisol plays a critical role in sleep-wake cycles: stimulating wakefulness in the morning, continuing to support alertness throughout the day, while gradually dropping to allow the body’s own internal sleep drive and other hormones—including adenosine and melatonin—to rise and help bring about sleep.
This evening-low, morning-high daily cortisol rhythm is true for most chronotypes: Lions, bears, and wolves. In dolphins, however, the cortisol rhythm is inverted. Dolphins have cortisol on the rise at night and reaching its lowest levels in the morning. This inverted cortisol rhythm contributes to the difficulty falling asleep, restless and light sleep, groggy mornings, and daytime fatigue that is so common among dolphin chronotypes.
That’s cortisol and its rhythm in balance, or homeostasis. Too often, the cortisol rhythm is thrown out of sync, leading to problems with sleep and health. Cortisol levels can be too low, but much more often, it’s elevated cortisol that’s the problem.
Chronic stress is a major contributor to elevated cortisol, an excessively active HPA axis, and an ongoing state of arousal that’s exhausting, anxiety-producing, and sleep-depriving. As I’ve said, elevated cortisol also contributes to a compromised immune system, chronic inflammation, weight gain, and, eventually, to chronic disease.
Poor sleep itself also can increase cortisol production and dysfunction of activity along the HPA axis. Research shows that cortisol can be elevated by:
- Poor-quality sleep
- Lack of sufficient sleep
- Inconsistent sleep schedules (including rotating schedules adhered to by shift workers)
Research shows a complex two-way street between the HPA axis (which produces cortisol and regulates its levels) and sleep. Poor, insufficient, irregular sleep increases the activity of that system, leading to more stress, greater arousal, and, over time, to the health complications I’ve mentioned above. And a more active HPA axis can interfere with the ability to maintain consistent sleep routines and to get enough sound, high-quality sleep. It can be a vicious cycle.
What to remember: Cortisol production follows a daily, 24-hour biorhythm, lowest overnight and highest first thing in the morning. When that rhythm gets disrupted, sleep does too.
Michael J. Breus, Ph.D., DABSM
The Sleep Doctor™