Feeling Sick and Tired? Here’s Why You Get Tired When You’re Sick

why you get tired when you're sickYou’ve probably never given much thought to why you feel tired when you are sick—it just seems natural. When we are sick, our energy levels seem to drop and all we want to do is go to bed and sleep so we can feel better faster. The truth of the matter is not so much that we have less energy, but that our bodies demand more energy to mount an effective immune response. This energy requirement may be one reason why we feel fatigued when we have an infection.

Your Immune System Draws Energy When You’re Sick

When we are sick and have an infection, one of the things that our immune system does is to create immune cells and proteins, such as cytokines and antibodies, to fight the infection. The generation of immune cells is called clonal expansion. As is to be expected, our bodies need fuel to support this growth of these cells and the production of proteins. Recent research supports the notion that our immune system needs energy to function properly. In animal studies, animals that were fighting infections had a less robust immune response when their calorie intake was reduced. This intuitively makes sense: if we want to have the energy to fight an infection, we need to eat. Other studies indicate that oxygen consumption goes up in animals when the immune system is triggered. Again, oxygen is a primary requirement for the creation of energy in our bodies, so it stands to reason that when we use more energy, oxygen intake increases. We see correlations of this in people: malnourished children get sick more often, have more frequent infections and those infections tend to be chronic.

Immune Response Modifiers Can Also Make You Tired

In addition to increased energy requirements, certain components of the immune response can actually make you feel tired and sleepy (i.e., they are somnogenic). For example, interleukin-2 and interleukin-15, two proteins produced during an immune response, can increase non-REM (non-dreaming) sleep in animals. Interleukin-1-beta and tumor necrosis factor-1, another two components of immune response, are important sleep regulatory substances in the brain. So not only does our immune system take up more energy, it actively produces substances that make us tired when we’re sick.  Conversely, sleep loss dampens your immune system—so if we are tired, we may be at a greater risk for getting sick. We have all had periods where we felt run down or worn out and maybe we’ve noticed a higher propensity to come down with something after this.

Rest Helps Us Fight Infections

It makes sense that the immune system would promote sleep when we are fighting off sickness. When we sleep, our bodies enter a lower energy state. When we are sick, our energy resources can be focused on producing a robust immune response. Also, if we are in bed, that means we are not out and about—reducing the chance that we run into another infection while we are still fighting off the first infection.

Relax! Being Sick and Tired is Only Natural

So, give in when you are sick and tired—let your body do what it is designed to do. Get the rest you need so you can get back on your feet more quickly. Make sure you give it the energy it needs by taking in enough calories. And of course, the best way to keep yourself active and energetic is to avoid getting sick in the first place. And healthy immune function goes a long way to help us avoid getting sick.

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