The Time Of Day Actually Makes A Difference To How We Get Sick

If you’ve ever felt like your body is more likely to succumb to infection in the evening, then you’ve got your internal biological clock to thank.

Scientists have discovered that the human body’s response to being infected with a parasite differs depending on whether it is the morning or the evening, because our immune system reacts differently depending on the time of day.

Therefore the severity of any infection we contract will vary depending on when exactly the parasite enters our body.  

Professor Nicolas Cermakian, lead author on the study, has previously conducted research that showed the immune system has its own biological clock.

“Our body’s defence mechanisms are more or less active at different times of the day,” he explained.

To test the theory, Cermakian’s team from McGill University and the Douglas Mental Health University Institute, looked at a specific parasite that causes Leishmania’s infection and is transmitted via the bite of a female sandfly.

Infecting approximately 1 million people worldwide every year, with potentially deadly consequences, Leishmania is present in 90 tropical countries across Africa, South America, central and East Asia, and (thanks to climate change) now in Europe.

It presents as a skin lesion that develops weeks or months after infection and can also cause the lymph nodes to swell. If left untreated these sores persist for several weeks.

The researchers found that Leishmania’s infection was more effective at infecting humans in the early evening hours.

“We already knew that viral and bacterial infections were controlled by our immune system’s circadian rhythms, but this is the first time this is shown for a parasitic infection, and for a vector-transmitted infection,” said Cermakian.

Once they’d established when the parasite was being most successful, they looked at the correlation with the immune system, and surprisingly found that the early nighttime was also when the immune response to the parasite was the strongest.

So why would the parasite be transmitted by a fly that bites at the exact time when our defences are at their strongest?

Simply put, the parasite thrives when it elicits a strong immune response as it attracts inflammatory cells (known as macrophages and neutrophils) to the infection site, which it uses to multiply.

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Despite the seemingly paradoxical reasoning, knowing that the parasite interacts with the body differently at different times in the day, could help pave the way for new treatment and prevention for parasitic infections, say the team.

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