On a warm Parisian evening around 1729, the Seine river snailed past the Institut de France, inside which polymath Jean-Jacques Dortous de Mairan fixated on the slow movements of a plant (1). The fern-like leaves of his Mimosa pudica spread wide toward the sun during the day. Yet at night, the leaves furled back inward as if to sleep. 

Dortous de Mairan intervened. He stowed the plant in the dark, wondering whether the cycle would hold. It did (2). Even without absorbing sunlight, the mimosa carried out its daily rhythm. 200 years passed before biologists appreciated the discovery as an internal clock and coined the term “circadian rhythm.” 

“For a few centuries, people interested in circadian rhythms were mainly botanists,” said Nicolas Cermakian, a chronobiologist at McGill University.

Today, scientists understand the importance of daily rhythms. The human circadian system regulates sleep and the function of every tissue in the body. All organs and cells throughout the body have their own internal clocks, which cycle between different functions such as assembling particular proteins and receiving molecular messages. Disruptions like sleep deprivation, shift work, and even jet lag can deteriorate health by increasing the risk of metabolic disorders, cardiovascular disease, and cancer, and scientists’ understanding of human rhythms is rapidly evolving (3).

“The science is relatively new,” Cermakian said. “There was almost nothing on immunity and circadian rhythms until the year 2000.”

Immune function, it turns out, ebbs and flows daily. Mice mount weaker immune responses at different times. In humans, proper timing makes some vaccines more effective. Researchers have often found that immune cell counts peak when we rest at night (4). These discoveries have prompted clinical scientists to explore how to wedge immune chronobiology into practical advice for treating disease. Already, they’ve shown that administering cancer drugs at a particular time of day can extend life expectancy by a few years in some cases (5). 

Hopes are high. “Every life saved, or every year that you gain is so precious,” Cermakian said.  Yet, as the wave of experiments unmasks immunological rhythms at the molecular scale, researchers envisage profound innovations in how to administer drugs — and when. 

Zeroing in on immunity

The first hints of a circadian immune system came in 1960 when biologists discovered a “susceptibility rhythm” in mice dosed with toxins (6). The same animal appeared far more vulnerable, depending on the time of day. “If you inject a certain pathogen in the evening in the mouse, the animal will die,” said Andrés Hidalgo, an immunologist at Yale University with an interest in circadian biology. “If you give the same dose during the day, sometimes it doesn’t even notice.”

Immune response oscillates in a circadian rhythm partly because the levels of immune cells (or leukocytes) do too. 

Biologists first proved that inflammation fluctuates in mammals’ innate immune systems — the fast, nonspecific arm of immunity that includes macrophages, neutrophils, and dendritic cells (7,8). Sixteen years later, they found the same in the adaptive immune system, which forms memories to destroy specific threats with B and T cells (9).

The benefit of obeying circadian rhythms in medicine soon became clear. Cardiologists in the 1980s noticed that heart attacks and death correlate with time of day (10,11). “They were always in the early morning. If you plotted all the thousands of patients that came to the hospital, you could draw an almost perfect sinusoidal curve,” Hidalgo said.  

As biologists uncovered rhythms in immune responses, they were in awe of how many conditions these rhythms could affect. In a 2006 study, scientists demonstrated that mice with broken rhythms died younger and aged faster, developing sarcopenia, cataracts, and shrunken organs (12). For anything from viral infections to rheumatoid arthritis to breast cancer, circadian immunity seemed to matter.

The mysteries of immune rhythm

Biologists like Cermakian have begun to pick apart why immune function varies throughout the day. “Now that we know that the clocks are controlling most of our physiology, we need to understand how in order to harness all applications that we may think of,” he said. 

In mouse studies, researchers tinkered with genes, proteins, and environmental conditions. Circadian clocks control how mobile leukocytes are and how populous they are in certain organs. What’s come to light is just how profoundly rhythmic each type of immune cell is. 

Mast cells, which contribute to allergic reactions, change their function throughout the day (13). Cermakian’s team has shown a daily rhythm in how much T cells proliferate after stimulation (14). The surface proteins that immune cells use to propagate messages are more abundant at particular times of day, depending on the cell (15). Dendritic cells, which relay information from the innate to the adaptive immune system, migrate more during rest. This “dendritic clock” controls how efficiently a body can clear a parasite (14). All of that up and down creates daily oscillations in proinflammatory compounds too. Macrophages produce more inflammatory cytokine molecules during rest (16).

These patterns help scientists understand the daily burdens of inflammatory diseases. Rheumatoid arthritis may flare due to surges of cytokines. “Talk to people who suffer from arthritis, for example; they will tell you that they have more pain, stiffness, and so on in the morning,” said Cermakian. Asthma hits hardest very late at night, perhaps because that’s when cortisol crests (17). Other diseases likely have similar dependencies. “And when we know that,” Cermakian said, “then we'll be able to maybe act on those mechanisms so that we can control the disease better.”

That’s easier said than done, Hidalgo noted. The circadian clock regulates about a third of the transcriptome, the cell’s big recipe book of proteins. “It's going to affect everything in metabolism, cancer, proliferation, migration,” he said. 

Hidalgo has primarily studied rhythms of neutrophils, innate immune cells that gobble up invaders. Neutrophils usually live for less than 24 hours, yet he’s uncovered a long list of rhythmic daily behaviors (18). 

“There were changes in numbers. There were changes in the genes. There were changes in protein content and granules that execute functions,” he said. Neutrophils aggregate into neutrophil extracellular traps that capture microbes. This defense is much more efficient during rest (19). Although most detailed studies of circadian immunity deal with mice, recent human studies revealed that the same lessons generally apply (7).

Chronomedicine on the horizon

Circadian immunity experts believe that the discoveries to date are just the tip of the iceberg. 

For some researchers, the next step is to test for the best time to administer drugs. An effective vaccination program, for example, should maximize a person’s immune memory. Cermakian’s team reported that the T cell responses to certain vaccines are very active during the day. Molecules that limit T cell response in these cases are more active at night.

“This explains that when we do a vaccination in the night, there's less of a response,” Cermakian said.

Immunologists have studied how time of day boosts vaccination for influenza, SARS-CoV-2 and tuberculosis (8). Each produced stronger responses when administered in the morning, rather than afternoon or evening. Morning influenza jabs led to greater antibody responses; morning tuberculosis shots rallied more cytokines and stronger immune memories; and SARS-CoV-2 vaccines summoned more antibodies, dendritic cells, B cells, and T cells.

“If selecting the right time of day will make the vaccine two or three times more efficient, that can be a lot,” Cermakian said “That can save many lives.”

If selecting the right time of day will make the vaccine two or three times more efficient — that can be a lot. That can save many lives. 
- Nicolas Cermakian, McGill University

With cancer drugs, there’s usually a time that leads to more efficient treatment and fewer side effects. Cancer progression follows circadian rhythms too. In a breast cancer study, researchers found that metastasis accelerates during sleep (20). According to Yool Lee, who studies translational medicine and physiology at Washington State University Spokane, if clinicians want to suppress cancer, it makes sense to drug it right when it propagates. “Nighttime treatment is probably better,” he said. 

The same logic holds for treating autoimmune diseases which often spike during sleep because of nightly surges of cytokines. “This will be true for many aspects of medicine and treatments and vaccination,” said Cermakian.

Drugging the rhythm

In the clinic, the true solution won’t be as simple as telling patients to take a pill before noon. “You have to determine that for each drug,” Cermakian said.

Then, suppose a clinical trial reveals that a new drug works best during a specific circadian phase. One can’t expect every person to be on the same cycle simultaneously, so doctors may have to control the rhythm itself. 

“Each individual person could be different,” said Lee. A patient may use blue-light therapy in the morning or avoid large meals several hours before treatment (21).  When light enters the retina, the brain’s central clock resets. “That ‘reset’ information transmits all over the body,” Lee said, including “the ‘liver clock’ or ‘kidney clock’, even the skin — all the tissues in the body.” 

Other factors nudge the timing of the body’s internal oscillations, though to a lesser extent: “Feeding, exercise, social or physical stress, even drugs or medications disrupt rhythms or rectify rhythms,” said Lee.

This means that clinicians could exploit the power of light, meals, exercise, and perhaps melatonin — a hormone produced in response to darkness that people can take as a supplement — to mobilize a patient’s immune system for maximal effect. 

Drugs administered this way will catch leukocytes in stride, allowing them to find their targets when there are more targets to find. 

Still, Cermakian cautioned that it’s too early to make sweeping statements such as recommending that people always schedule vaccines for the morning. 

“We have a tendency of assuming that the easy way of explaining is the one. No, it's always much more complex,” he said. “When you think you've found the answer to a question, there are ten new questions that arise.”

References

1. "Jean-Jacques d'Ortous de Mairan." Académie française. Retrieved March 7, 2023, from https://www.academie-francaise.fr/les-immortels/jean-jacques-dortous-de-mairan 
2. "The Birth of Chronobiology: A Botanical Observation." (n.d.). Society for Research on Biological Rhythms. Retrieved March 7, 2023, from https://srbr.org/the-birth-of-chronobiology-a-botanical-observation/ 
3. Reddy, A.B. and O'Neill, J.S. Healthy clocks, healthy body, healthy mind. Trends in Cell Biology  20, 36-44 (2010).
4. Wyse, C., O'Malley, G., Coogan, A. N., McConkey, S., and Smith, D. J. Seasonal and daytime variation in multiple immune parameters in humans: Evidence from 329,261 participants of the UK Biobank cohort. iScience  24, 102255 (2021).
5. Lee, Y. Roles of circadian clocks in cancer pathogenesis and treatment. Experimental & Molecular Medicine  53, 1529-1538 (2021).
6. Halberg, F., Johnson, E.A., Brown, B.W., and Bittner, J.J. Susceptibility Rhythm to E. coli Endotoxin and Bioassay. Proceedings of the Society for Experimental Biology and Medicine  103, 142-144 (1960).
7. Abele, S.H., Meadows, K.E., Medeiros, D., and Silver, A.C. Time is on the Immune System's Side, Yes it is. Yale Journal of Biology and Medicine  92, 225-231 (2019).
8. Wang, C., Lutes, L.K., Barnoud, C., and Scheiermann, C. The circadian immune system. Science Immunology  7, eabm2465 (2022).
9. Fernandes, G., Halberg, F., Yunis, E.J., and Good, R.A. Circadian rhythmic plaque-forming cell response of spleens from mice immunized with SRBC. Journal of immunology  117, 962–966 (1976).
10. Muller, J.E. et al. Circadian variation in the frequency of onset of acute myocardial infarction. The New England Journal of Medicine  313, 1315–1322 (1985).
11. Muller, J.E. et al. Circadian variation in the frequency of sudden cardiac death. Circulation  75, 131–138 (1987).
12. Kondratov, R.V. et al. Early aging and age-related pathologies in mice deficient in BMAL1, the core component of the circadian clock. Genes & development  20, 1868–1873 (2006).
13. Nakamura, Y. et al. Circadian regulation of allergic reactions by the mast cell clock in mice. The Journal of Allergy and Clinical Immunology  133, 568–575 (2014).
14. Fortier, E. E. et al. Circadian variation of the response of T cells to antigen. Journal of Immunology  187, 6291–6300 (2011).
15. Silver, A.C., Arjona, A., Walker, W.E., and Fikrig, E. The circadian clock controls toll-like receptor 9-mediated innate and adaptive immunity. Immunity  36, 251-61 (2012).
16. Gibbs, J. E. et al.  The nuclear receptor REV-ERBα mediates circadian regulation of innate immunity through selective regulation of inflammatory cytokines. Proc Natl Acad Sci U S A  109, 582–587 (2012).
17. Habbal, O.A. and Al-Jabri, A.A. Circadian rhythm and the immune response: A review. International Reviews of Immunology  28, 93-108 (2009).
18. Casanova-Acebes, M. et al. Rhythmic modulation of the hematopoietic niche through neutrophil clearance. Cell  153, 1025–1035 (2013).
19. Ella, K., Csépányi-Kömi, R., and Káldi, K. Circadian regulation of human peripheral neutrophils. Brain, Behavior, and Immunity  57, 209–221 (2016).
20. Diamantopoulou, Z. et al. The metastatic spread of breast cancer accelerates during sleep. Nature  607, 156-162 (2022).
21. Gooley, J.J. Treatment of circadian rhythm sleep disorders with light. Annals of the Academy of Medicine  37, 669–676 (2008).