Almost all living things have at least one form of defence that works to ward off disease-causing organisms. In both vertebrates and invertebrates, an important physiological reaction known as the innate (or natural) immune response occurs when a being comes into contact with viruses, bacteria, toxins, or other foreign matter that could threaten its health if not removed from the body. Innate immunity is non-specific, meaning it is directed at any pathogen that enters the body and doesn’t distinguish between specific invaders.

Only vertebrates possess adaptive (or acquired) immunity, a more sophisticated network of defence mechanisms that can identify and eliminate specific substances that put the host organism’s health at risk. Even the world’s most primitive vertebrates who, from an evolutionary standpoint, have remained largely unchanged for hundreds of millions of years — like the lamprey, an ancient species of eel-like, jawless fish — are capable of a weak but demonstrable immune response.

Humans have an incredibly complex immune system, comprised of organs, cells, chemicals, and antibodies that collaborate to protect the body from infection and disease. The human immune system also helps to monitor and eradicate abnormal or damaged cells, including cancerous ones.

The immune system plays a vital role in our overall health, and like many other facets of our being, it can be affected by how well — and how much — we sleep.

What happens in the immune system during sleep?

As we sleep, our bodies undergo a plethora of processes that regulate and strengthen the immune system. Getting a sufficient amount of high-quality sleep supports innate and adaptive immunity, helping the body to stave off infection and disease. Many immune functions display rhythms in synchronicity with the 24-hour sleep-wake cycle, and certain immunological processes have been linked to specific sleep stages.

During sleep, the body releases cytokines, small proteins associated with inflammation that aid in cell-to-cell communication. These cytokines are responsible for the growth of blood cells and other cells involved in immune function, and they enhance the immune system's ability to combat infections and repair wounds. This inflammatory process takes place even when a person isn’t injured or sick, indicating that cytokines play a role in bolstering adaptive immunity.

Research suggests that sleep is also important in building immune memory. To perform to the best of its function, the immune system must know how to perceive and react to antigens (toxins or other foreign matter that can cause harm). Through the interaction of its components during sleep, the immune system’s ability to recall how to recognize and respond to these antigens is reinforced.

It isn’t entirely clear why the immune system carries out these activities during sleep, but a range of factors are thought to be involved. As we sleep, breathing slows and muscle activity levels drop, which may allow the immune system the additional energy it needs to perform its tasks. 

It is also theorized that the human body has evolved to let these immunological processes take place while we are asleep because the inflammation produced by them could affect physical and mental performance if it were to be present during waking hours. Similarly, the sleep-promoting hormone melatonin — which is released from within the brain at night — helps alleviate the stress that can come from inflammation during sleep.

Does sleep deprivation affect the immune system?

Research suggests that sleep deprivation can be detrimental to immune function. In a healthy sleep cycle, inflammation that occurs throughout the night as a result of immunological activity tapers back to a normal level before a person wakes up. This self-regulating system fails when a person is sleep-deprived, causing inflammation to persist even after they have awoken. Studies indicate that the immune system doesn’t become accustomed to a lack of sleep over time, meaning that low-level inflammation can continue and become chronic in those who regularly miss out on sleep.

People who consistently get less sleep are more likely to catch infections after being exposed to viruses, including the common cold. Lack of sleep is also connected with a number of longer-term illnesses, including diabetes, cardiovascular disease, and neurodegenerative diseases, and this is thought to be due to sleep deprivation’s negative effects on the immune system.

Just as sleep — or a lack thereof — can affect the immune system, immune system activity can also interfere with sleep. It’s normal for a person to become tired or lethargic when they are dealing with an illness. The body uses a considerable amount of energy to fight sickness, and feelings of sleepiness may be its way of telling a sick person to rest and let the immune processes that take place during sleep begin.

Getting enough good-quality sleep can help bolster immunity and reduce the risk of illness, providing a vital line of defence against infection and disease. Although sleep is integral to the proper function of the immune system, it’s important to note that eating well, maintaining a healthy weight, being physically active, not smoking, and avoiding excessive alcohol consumption are also key in supporting immune health.

Sleep disorders (such as insomnia and sleep apnea) can have an impact on immune function. People who experience severe or chronic sleep issues with recurring illnesses should consult a doctor, who can work to identify the underlying cause and provide guidance and treatment.