DiscourseJuly 6, 2021 10:53:28 IST
When microorganisms – like bacteria or viruses – infect us, our immune system jumps to action. It is highly trained to detect and remove infections and the damage they cause.
It is typically assumed that our immune system works in exactly the same way regardless of whether the infection occurs during the day or at night. But more than half a century of research now shows our bodies answer differently day and night. The reason for this is our body’s clock and the fact that every cell in the body, including our immune cells, can tell what time it is.
Our body clock has evolved over millions of years to help us survive. Every cell in the body has a collection of proteins that show time depending on their level. Knowing day or night means that our bodies can adjust their functions and behaviors (like when we want to eat) at the right time.
Our body clock does this by creating 24 hour rhythms (also called circadian rhythm) how cells work. For example, our body clock ensures that we only produce melatonin when the night goes down because this chemical is tiring us – which means it’s time to sleep.
Our immune system is made up of many different types of immune cells that constantly patrol the body in search of evidence of infections or damage. But our body’s clock determines where these cells are located at certain times of the day.
Generally, immune cells travel to the tissues during the day and then circulate around the body at night. This circadian rhythm of immune cells may have developed so that the immune cells are located directly in the tissues at the time we are more likely to be infected, able to attack.
At night, immune cells circulate around the body and stop in our lymph nodes. Here, they build a memory of what they encountered during the day – including possible infections. This will ensure that they can respond better to infection next time they encounter it.
Given the control of the body clock in our immune system, it is not surprising to learn that some studies have shown that time is infected virus – like influenza or hepatitis – can affect how we get sick. The exact timing is likely to vary depending on the virus in question.
Other studies have also shown that the timing of taking medicines can affect how they work – but this again depends on the medicine in question. For example, because we make cholesterol when we sleep, we take a short-acting statin (a cholesterol-lowering drug) just before bedtime offers the most benefit. It has also been shown that the time of day affects how well certain types of immune cells work.
Body watches and vaccines
There is also growing evidence that vaccines – which create the memory of the immune system of a particular pathogen – affect our body’s clock and the time at which the vaccine is given.
For example, a randomized study in 2016 of more than 250 adults over the age of 65 found that influenza vaccine in the morning (between 9.00 and 11.00) resulted in a higher antibody response compared to those vaccinated in the afternoon (15.00–17.00).
Recently, in the mid-20s, people immunized with the BCG (tuberculosis) vaccine from 8 a.m. to 9 p.m., had an improved immune response compared to those vaccinated between noon and 1 p.m.. So there is evidence in certain vaccines that early morning vaccinations can provide a more robust response.
One reason for improving the immune response to vaccines in the morning may be due to the way our body clock controls sleep. In fact, studies have shown that adequate sleep after vaccination for hepatitis A. enhances the immune response by increasing the number of vaccination-specific immune cells that provide long-term immunity compared to those whose sleep had been restricted after vaccination.
It is not yet fully understood why sleep improves the vaccine response, but it may be due to how our body’s clock directly controls the function and location of immune cells during sleep. So, for example, it sends immune cells to our lymph nodes while we sleep to learn about daily infections and build a “memory” of this.
Of course, this raises the question of how all of this can relate to current pandemic and global vaccination programs. How are we immune system clock works can be important whether we are developing COVID-19. Interestingly, the receptor that allows the COVID virus, SARS-CoV-2, to enter our cells is under the control of our body’s clock.
In fact, levels of this receptor are higher in the cells that line our airways on different days. This may mean that we are more likely to get COVID-19 at certain times of the day, but more research is needed to find out.
Does the immune response affect whether the time has been vaccinated against COVID-19 without answering. Given the high efficiency of many Covid-19 vaccines (over 90 percent efficacy reported by both Pfizer and Moderna) and the urgency with which we need to vaccinate, people should be vaccinated at any time of the day for them.
But current and future vaccines that are less effective – such as the flu vaccine – or if they are used in people with a weaker immune response (such as older adults), a more precise “timed” approach can ensure a better immune response.
Annie Curtis, Senior Lecturer, Medicine and Health Sciences, RCSI University of Medicine and Health Sciences