In this edition, we're diving into a specific facet of the sleep-wake cycle: the relationship between cortisol and sleep.
Cortisol, a hormone produced by the adrenal glands that sit atop the kidneys, plays several roles in the human body, including regulating blood sugar, blood pressure, and body temperature. The hormone is best known for its role in the stress response — aka “fight-or-flight” mode — a physiological reaction to a perceived threat that triggers a cascade of changes in the body to help us survive.
However, one of its lesser-known roles is helping to control the sleep-wake cycle in our bodies. It turns out this hormone helps regulate sleep in several ways, both directly and indirectly.
During the day, cortisol helps keep us awake by suppressing the production of melatonin, a hormone that encourages sleep. At night, it helps keep us asleep by suppressing wake-promoting neurotransmitters, like dopamine and norepinephrine, and by stimulating the release of GABA, a neurotransmitter that makes us feel calm and relaxed. When night falls, our bodies produce less cortisol, which allows melatonin to take its place as the dominant hormone in the brain and facilitates sleep.
As we know, sleep is essential for health. It's the time when our bodies and brains recover and repair themselves after a long day of activity. Whether it’s lack of sleep or a high-pressure deadline at work, our bodies often call upon cortisol to help us deal with stressful situations. Chronic sleep deprivation can cause elevated cortisol levels, have detrimental effects on our body's ability to maintain homeostasis, and even disrupt the immune system.
At the same time, sleep can play a critical role in helping us manage our cortisol levels. Cultivating healthy sleep hygiene can lower cortisol, reduce the risk of physical and psychological illness, and improve sleep quality.
Read on to learn more about the relationship between cortisol and sleep and what you can do to keep them in harmony.
What Is Cortisol, and Why Should I Care?
If you've ever felt stressed, you've probably experienced the symptoms of cortisol. Also known as the “stress hormone,” cortisol is a steroid hormone produced in the adrenal glands in response to stress, exercise, and low blood sugar. The primary function of cortisol is to mobilize the body's resources, such as glucose and fat, to deal with a stressor.
When you think of stress, you probably first think of the physical manifestations: an increased heart rate, tight chest, clenched jaw, sweaty palms. The action of stress hormones, such as cortisol, is responsible for these physical symptoms — a series of chemical reactions that prepare the body for a “fight-or-flight” response.
Cortisol is released by the adrenal glands in response to a stressful event or situation, but it's also released in response to the anticipation of a stressful event or situation. Both acute and chronic stress are known to increase the level of cortisol in the bloodstream, which, in turn, affects several areas of the brain and body.
How Do Sleep Cycles Work Again?
Sleep cycles are regulated by the hypothalamus, which maintains the circadian rhythm, often referred to as the body’s “biological clock.” This is the same part of the brain that modulates our body temperature, hunger, thirst, and the release of certain hormones.
The hypothalamus controls body and sleep patterns by sending out signals and transmitting them to many parts of the body. The pineal gland is another critical part of the brain that helps regulate the sleep cycle by releasing melatonin, a key hormone that dictates the body’s sleep-wake cycle.
When you’re asleep, you progress through different sleep stages, each associated with a specific physiological state and set of behaviors, such as brainwave patterns and muscle activity. As you advance through the stages of sleep, your brainwave patterns become slower, and you cycle through a repeating process of two distinct sleep states: NREM (non-rapid eye movement) and REM (rapid eye movement). The sleep cycle is a repeating pattern of non-REM and REM sleep, which usually occur about four or five times during a typical night's sleep. (Source)
When you first fall asleep, you typically spend about 90 minutes in an NREM phase before entering a short period of REM sleep and begin dreaming. During REM sleep, your breathing and pulse increase and your body becomes paralyzed (which is an evolutionary safety mechanism in place to prevent you from acting out your dreams!).
Non-REM sleep is divided into three stages:
- Stage one is a light sleep, characterized by theta waves in the brain, which occurs as you drift from being awake to falling asleep.
- Stage two is a phase of deep sleep where you spend about 50% of your sleep cycle. While you may not be consciously aware of your physical state, your body transitions into this state by dropping your body temperature, relaxing your muscles, and slowing your brain waves. (Source)
- Stage three is the deepest sleep (aka slow-wave sleep), characterized by delta waves. Stage three is also the stage in which night terrors, sleepwalking, and bedwetting most often occur.
What Does Cortisol Have to Do With Sleep?
Sleep and cortisol are part of the “HPA axis”
The hypothalamus, the pituitary gland, and the adrenal glands work together in what is known as the hypothalamic-pituitary-adrenal (HPA) axis. Feedback from the hypothalamus and the pituitary gland influences the production of cortisol by the adrenal gland. Psychological and physical stressors activate the HPA and stimulate increased production of cortisol, which interferes with normal sleep. Because sleep inhibits the HPA and cortisol secretion, sleep deprivation becomes a further stressor promoting cortisol production. (Source, Source)
Sleep and cortisol possess a bidirectional relationship
Research has revealed that a two-way (bidirectional) relationship exists between sleep and cortisol, resulting in a vicious cycle when the body is thrown out of homeostasis. Lack of sufficient sleep, low-quality sleep, and inconsistent sleep schedules (as seen in night-shift workers) can all contribute to elevated cortisol levels. Elevated cortisol levels then contribute to an overactive HPA axis interfering with our ability to get enough high-quality sleep. Studies have shown that elevated levels of cortisol during sleep increase brain activity, reduce the amount of slow-wave sleep, and increase wakefulness throughout the night. (Source, Source, Source)
Sleep and cortisol both follow a circadian rhythm
Like sleep, research has shown that cortisol levels follow a diurnal rhythm (i.e., they rise and fall with the sun) and are typically highest in the early morning and lowest in the evening. According to studies, the lowest cortisol level typically occurs around midnight, begins to rise two to three hours after onset of sleep, and peaks about 45 to 60 minutes after you wake up in a phenomenon known as the “awakening response.” In this way, cortisol plays an important role in sleep-wake cycles: after encouraging wakefulness in the morning and alertness during the day, it declines in the evening so melatonin levels can rise and promote sleep. (Source)
Sleep deprivation raises cortisol levels
When we don’t get enough sleep, our bodies tend to produce higher levels of cortisol. Sleep loss is considered a physiological stressor that can over-activate the HPA axis, causing the hypothalamus and pituitary gland to signal the adrenal system to constantly produce more cortisol. Some studies have shown elevated cortisol levels the evening after a single night of sleep deprivation, although these results have not been consistent. (Source)
Sleep and cortisol affect our metabolism
Studies show that sleep deprivation can increase circulating cortisol levels, thereby increasing blood sugar levels and decreasing insulin sensitivity. Researchers have also found a correlation between short sleep duration and an increased risk of developing obesity and type II diabetes. One long-term study followed nearly 500 participants from age 27 to age 40, finding that those who reported short sleep duration also tended to experience more weight gain and obesity. These findings were supported by a second study that looked at nearly 1,000 participants. This research suggests HPA axis hyperactivation may be one of the biological mechanisms responsible for the metabolic consequences of sleep loss. (Source, Source)