A basic internet search of the word “hormones” might have you think that their main functions are to cause pubertal embarrassment, make females moody, and get professional athletes into trouble. However, there are many more types and functions of hormones besides those involved in sexual characteristics. Hormones are important for our metabolism, stress response, tissue repair, energy levels, sleep, and the list goes on. 

Simply, hormones are small molecules used by our organs to communicate over long distances. For example, when your body needs energy, your brain needs to receive that information in order to make you engage in eating behavior. So, the empty stomach releases the hormone ghrelin, which travels to the brain with the chemical message, “Make the organism eat something nutritious”. But since your brain is now involved, you eat gummy bears instead. 

Since the body’s needs change throughout the day and night, so do the hormonal messages. Certain hormones have seesaw relationships with other hormones—as one decreases activity, the other increases, and vice versa. This healthful balance is influenced significantly by our sleep-wake cycle. As described in our article “What is Melatonin?” , melatonin is a hormone that helps our body “know” what time it is. If our sleep patterns don’t match our hormonal cycles, then we can experience suboptimal health conditions. Before we get into the influence of sleep on our hormones, let’s introduce the system of the body in which they act: the endocrine system. 

The great communicator: The endocrine system

The endocrine system is made up of organs that secrete hormones and their target tissues. Hormones are small molecules that are made in one tissue and released into blood circulation to have an effect on another tissue. Once a target tissue receives the hormonal signal, it typically releases another hormone to send to a different target tissue. This is how the endocrine system uses hormones to communicate physiological needs very quickly throughout the body.

Back to our earlier example, the empty stomach secretes a hormone (ghrelin) whose target tissue is the brain. When the brain receives this signal, it releases a hormone whose target tissue is the adrenal glands. When the adrenal glands receive this signal, they release a hormone whose target tissues are in the gastrointestinal tract, brain, and skeletal muscle. This (very simplified) process orchestrates when and how you seek out food, eat, drink, digest, stop eating, run from bears, excrete waste, reproduce, sleep, and start all over. In that order, of course.

Almost all hormones display circadian rhythmicity, which means that their levels rise and fall over a 24-hour cycle. Just a few of the hormones that display this activity include cortisol, growth hormone, thyroid hormone, testosterone, and melatonin. 

Not only is the production and secretion of hormones controlled by the sleep/wake cycle, but also is the sensitivity of target tissues to hormonal stimulation. This means that, for endocrine signals to work, the hormone needs to be released at the same time as its target tissue is sensitive to it. We discuss this more in our article “Sleep and Biological Clocks”. 

Stress and Repair

Certain parts of the brain (the hypothalamus and pituitary gland) and the adrenal glands use hormones to work together in what is called the HPA axis. The HPA axis is most often associated with the stress response, but it’s also important for regulating our metabolism and maintaining healthy tissues. The term “stress response” may sound like a bad thing, but it actually helps us restore homeostasis after we experience stresses from the environment and from within our bodies.¹ The main hormone involved in the stress response (and other functions of the HPA axis) is the adrenal hormone, cortisol.

Even in the absence of stress, cortisol naturally rises and falls throughout the day and night. Cortisol secretion is lowest at night while we’re sleeping, then peaks in the morning to help facilitate our transition to wakefulness and activity. Importantly, sleep has significant effects on our cortisol levels. Multiple studies have shown that poor sleep increases the amount of cortisol secretion in response to stress; in other words, without good sleep, your stress response becomes more reactive.² This greater stress reactivity initiates a destructive cycle, in which higher cortisol levels make it hard to fall asleep, so you get poor sleep because of your enhanced stress response.^2,3 Scientists have yet to determine how this cycle affects the stress response in the long-term, but it is known that altered cortisol reactivity is associated with inflammatory diseases, such as obesity and cardiovascular issues.^4,5

Another important hormone secreted by the HPA axis is growth hormone. In adults, growth hormone is important for maintaining tissue homeostasis, whereby the cells that compose our tissues are replaced due to damage or age. Growth hormone is also important for glucose metabolism; it keeps our blood glucose levels steady in the absence of food. At night, growth hormone secretion reaches its peak, when it helps provide energy to our tissues while we’re fasting. 

Energy Metabolism

During sleep, you’re normally not eating. (Except in your dreams, especially if you’re restricting…) But the body still requires energy to function! Incredibly, our energy needs only decrease by 15% during sleep.⁶ In order to maintain metabolic homeostasis while we’re sleeping, two main things need to happen: 1) our blood glucose levels need to remain steady, and 2) we need to break down fat to use for energy instead of glucose. 

Our hormones facilitate these needs by adjusting their levels while we sleep: 

• Cortisol becomes low, which decreases our energy requirements.
• Growth hormone becomes high, which prevents a drop in blood glucose. 
• Insulin becomes low, which allows fats to be used for energy.
• The hormone leptin is secreted from adipose (fat) cells, which suppresses appetite.

In the early morning, the body anticipates the increased energy requirements upon waking by ramping up secretion of hormones that were low in the middle of the night. There is a rise in blood glucose concentration, followed by a drop in growth hormone, which allows insulin to rise in response to high blood glucose. This is called the “morning phenomenon”, and it happens in people with normal blood glucose, but it’s exaggerated in those with diabetes.⁷ This early morning jumpstart of glucose and insulin coincides with the improved glucose tolerance and insulin sensitivity experienced by many diabetics in the morning, versus in the evening.⁸

The timing of these hormones and the sensitivity of the whole body to insulin is normally coordinated by the secretion of cortisol from the HPA axis.⁹ As you learned earlier, sleep quality directly affects how much cortisol is secreted in response to mental and physical stresses. This means that poor sleep, potentially through its effects on cortisol, can negatively affect your metabolism. Learn more about how unhealthful sleep/wake behaviors affect energy metabolism in our article, “Sleep and Biological Clocks”.

In closing

Hormones are the signals our body uses to communicate between organs. Depending on the time of day, our organs need different types of information. The hormones that coordinate stress, repair, and energy metabolism cycle over a 24-hour period in order to facilitate tissue homeostasis. But hormones also respond to our behavior, for better or worse! With poor sleep, our hormones respond in acutely adaptive ways, but these adaptations are usually unhealthy in the long run. Conversely, beneficial sleep behaviors allow our hormones to be a crucial part of our toolkit for optimal health.