01/23/2017 03:15 EST | Updated 02/22/2017 03:02 EST

Why Colds And The Flu Feel Worse At Night

KatarzynaBialasiewicz via Getty Images
A lot of medicines on night table in bedroom

As cold and flu season continues to rage across the country, many Canadians are dealing with a rather frustrating conundrum. During the day, they seem to fare well with the infection but as soon as bedtime arrives, the situation worsens. The aches and pains return, the nasal passages fill up with mucus, and that infernal coughing comes back with a vengeance.

This change in symptoms is not due to the virus, however. Instead, this affliction is due to a normal phenomenon occurring each and every day inside us. Much like our own active and rest cycles, our natural defense force, the immune system, has its own daily - or circadian - rhythm.

Immunity is a 24/7 process. It's constantly at work to keep us safe from infections. But researchers have known for decades the type of immune response varies over the course of a twenty-four hour day.

During the day, one branch of the immune system called cellular immunity is at the forefront of the defenses. As we get ready to rest at night, another arm known as inflammation takes over. Usually, we don't feel the effects of cellular immunity. But we almost always know when we are inflamed as it promotes fever, mucus production, reduced breathing capability, and physical fatigue.

A few years ago, researchers uncovered a trigger of this shift in immune function. One of the major players in cellular immunity, T-cells, underwent a change such that the cells became less active against infective agents during our times of rest. It was as if the soldiers had recognized some kind of work whistle and decided it was time to rest.

This discovery reveals a rather strange trait of the immune system. When it comes to battling an infection, T-cells are incredibly important. They are the generals and the commanders of the operation. They simply should not be allowed to take a break. Yet no one has successfully explained why these cells tend to slow down when we need them the most.

Now it seems, we may know the reason. A team of German researchers have shown why these cells - and others - take that nighttime downturn in activity. The results show the answer isn't about rest, but ensuring the entire immune defense force is primed to fight in the morning.

The group used mice for their studies so they could better understand the link between the body's circadian rhythm and immune function. The mice were allowed to live as normal for weeks in a normal light-dark cycle. This would mimic a normal human day albeit in reverse as mice are most active during the night.

After the mice had grown used to the daily cycles, the housing stations for immune soldiers - the lymph nodes - were examined in order to determine any changes over the course of a twenty-four hour period. Although the team expected to see some difference, they were surprised to see such a stark change in population. During active times, the nodes were partially filled with cells. But when the mice rested, the nodes became filled with cells. As expected, T-cells were here but there was also another cell type crowding the environment.

It's known as a dendritic cell and one of its most important roles is to inform T-cells of any foreign activity. When a T-cell grabs a hold of this information, it can then initiate cellular immunity to tackle whatever threat may have arrived. The communication between these two cell types is critical to ensure a proper defensive posture and action.

This finding helps to reveal why we have this circadian switch in immune function became clear. During the day, both T-cells and dendritic cells are out and about, protecting the body and gathering valuable intelligence. As soon as the time for rest arrives, they head to the lymph node. Here, the dendritic cells share information with T-cells, who then develop an action plan for the morning. In the meantime, the inflammation arm of immunity is tasked with holding the fort.

From a military standpoint, this is an excellent strategy. The cellular commanders can congregate with the messengers and gain critical intelligence in a safe environment. This can improve the likelihood of a coordinated attack and increase the chances for a victory down the line.

Of course, if you happen to be fighting a cold or flu, this defensive tactic may be little more than cold comfort. After all, while the lymph node summits occur, you are left having to rely on inflammation to keep the infection at bay. This means those symptoms are guaranteed to occur and leave you in a dire situation for the evening and into the night. Just try to remember those meetings between dendritic cells and T-cells are for your benefit so that the infection can be defeated.

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