Numerous studies have demonstrated that many of the essential tasks of the immune system are governed by a 24-hour cycle of activity. In practical terms, this means that the time of day affects immune system function in important ways, including immune system cell activity and overall systemic threat response.

This 24-hour cycle, known as the circadian rhythm, has been with us, influencing our physical and mental functioning, for as long as we’ve been human. Indeed, all life on Earth is impacted by the approximately 24-hour cycle of day and night.

Circadian Rhythm and Cellular Activity

The overarching approximately 24-hour circadian rhythm impacts our overall functioning right down to the cellular level, with each individual cell organizing its activities according to this fundamental life rhythm.

With the fact in mind that chemical reactions are at the base of cellular activities, including metabolism and cellular waste removal, researchers through the years have discovered that there is a pattern to various cellular chemical levels and their resulting activities. At different times of the day or night, cells are performing different activities, typically in a regular pattern.

How Time of Day Affects Immune System Functioning

The working cells of the immune system are also subject to and influenced by these basic rhythms. A Rensselaer Polytechnic Institute study, for example, demonstrated that the activity of the killer cells of the immune system is directly linked to the circadian rhythm via the rhythmic timing of metabolism, a process that is also connected to the circadian rhythm.

Daily oscillation of key immune system cell substances, such as proteins and enzymes, was found to influence immune system bacteria and disease fighting responses, including body temperature and inflammatory response. Energy production within immune system cells is another essential cyclic process linked to circadian-rhythm-influenced timing.

One result of these cyclic processes is that they lead to relatively predictable patterns of activity. In addition to influencing the most fundamental cellular actions in the immune system, the time of day can even influence where immune cells are located in the body. For example, in the daytime, these cells tend to be concentrated in the tissues, ready to engage in protective action during what for most of the history of our species was the most active part of the 24-hour cycle. During that active period, people are more likely to get cuts and scratches, providing an opening for bacteria to enter the body, and more likely to be around a greater number of people outside of their family group, increasing their potential to be exposed to communicable diseases.

At night, however, these cells are found circulating throughout the body. Part of their journey includes visiting lymph nodes. Passing through the lymph nodes helps to enhance cellular memory via the communication of information the immune system has garnered through bacterial encounters and other interactions with potential agents of infection to immune system cells. This helps promote a more targeted immune response the next time the bacteria or other potential problem maker is encountered.

“When” Can Matter

Researchers have found that the severity of a viral infection can be impacted by what time of day the infection occurs. Viral replication rates can be impacted by immune system functions that help create a more or less favorable environment for the replication of such viruses as influenza and Covid-19. These immune system functions vary according to the daily cycle of cellular and systemic activity.

Working within that same concept, researchers have discovered that certain medications, like statins, and even some vaccines — including flu vaccines — seem to be more effective when given at certain times of the day, working with the natural activity rhythms of the immune system.

Circadian Rhythm and Health

Maintaining a healthy, minimally disrupted, circadian rhythm impacts health in a variety of ways. The circadian rhythm acts as a coordinating agent for the timing of countless essential bodily processes. It impacts the sleep and wake cycle. Sleep quality is also tightly intertwined with overall health and well-being, as well as with optimal immune function.

Circadian rhythms are an important driver of critical metabolic system functions and are linked to hormone production timing. Chronic circadian rhythm disruption, common among shift workers and overnight workers, has been linked to increased risk of a number of diseases, including heart disease, metabolic disorders and some cancers.

Better Rhythms, Better Health

Focusing on improving your body’s natural rhythms will help to promote optimal immune function and better overall health and well-being. Make lifestyle choices that result in a healthier circadian rhythm, such as keeping your waking, sleeping and meal times on a regular schedule.

Light is a powerful environmental cue for your circadian rhythm, so up your morning light exposure and decrease your bright artificial light exposure in the evenings for better sleep and a more natural circadian rhythm. Get enough physical activity every day, but do it earlier in the day, so as to not disrupt the rhythm of activity patterns to which your body evolved; evening workouts can be disruptive to your circadian rhythm. There’s no doubt about it; healthier life rhythms can result in better immune function and a healthier you.