Down with the Sickness: Why Do We Feel More Ill at Night?

by Rosalin Dubois

It is one of the worst times of the year - everyone, from your best friend to your professor, is getting sick. Sooner or later you probably will too, and when that time comes you’ll be struck by the same distressing experience: no matter how well you feel during the day, by the time you are ready to go to bed, you feel so miserable that you never want to leave your room again. I’ve always been told, “You’ll feel better in the morning; colds always feel worse at night!” But why is that true?

There are a few plausible explanations for this phenomenon. Some of these explanations are less scientific than others but may still be able to provide insight into why sleeping while sick can be so difficult. Consider everything that you have to distract you during the day. While you are focused on getting to class, meeting an important deadline, or even just socializing with your friends, you may pay less attention to the signals that your body is sending you that indicate you are sick. However, when you try to sleep, you have fewer distractions, and so you may notice more or these signals and feel much sicker. Additionally, when you lie down, gravity affects your body differently than when you are standing. Even just sitting up may help to clear your stuffy airways and help you to sleep better (essential for that 8:30 lecture!).

However, what if you are working late and sitting up, and still feel worse than during the day? You may be experiencing this because of the circadian rhythms of our immune system. Circadian rhythms are physical, mental, and behaviour changes that follow a daily cycle (think your “internal clock”) that tell you what time to get up in the morning. Our immune system also follows a similar pattern as researchers have found that immune response varies throughout the day. During the day, the part of the immune system called cell-mediated immunity (or just cellular immunity) is responsible for defending us from infection. This form of defence is very effective against viruses, bacteria, fungi, and other invaders. Most important to note is that we don’t typically feel the strain of this type of immunity at work.

At night, inflammation replaces cellular immunity. Inflammation is the type of immune response that we normally experience when our tissues are damaged by trauma, bacteria, heat, or other causes. When this happens, the cells that have been damaged release chemicals (including histamine and prostaglandins), which cause blood vessels to expand and allow more blood to reach the damaged area. Additionally, inflammatory mediators increase the permeability of blood vessels to defence cells that carry fluid into the tissue and cause swelling. By surrounding the damaging substance with a barrier of this released fluid, inflammation aims to isolate the invader from our tissues, and therefore prevent it from doing more damage. We experience the worst of our symptoms of being sick, like fever, increased amounts of mucus, and fatigue, when we feel the effects of swelling when our system is inflamed.

Studies over the last decade indicate that this transition between cellular immunity and inflammation occurs due to a change in the activity of a type of white blood cells called T-cells. These cells are important in cell-mediated immunity because they attack and kill antigens (foreign substances). It was determined that T-cells actually become less active against antigens during times when the body would normally be resting, especially at night.

This seems bizarre; why would the body turn off such important defenders when we need them most? A study done by a team of German researchers just last year may hold the answer to that question. In this study, researchers observed changes in the population of the lymph nodes of mice during their active times, and during their rest times. They correctly expected to see more T-cells present in the lymph nodes when they were not working and the mice were resting. However, they were surprised to find that high levels of dendritic cells were also present at this time. Dendritic cells process information about antigens and communicate this information to T-cells so that cellular immunity can effectively target this threat.
​
This research seems to indicate that during the day, T-cells and dendritic cells move normally throughout the body, gathering information and dealing with threats. When T-cells randomly come into contact with dendritic cells throughout the body, they receive information so that they can adapt their immune response to better eliminate the antigens. At night, both types of cell move to the lymph nodes, and the high concentration of these cells allows for a greater likelihood of interaction. Through this interaction, the T-cells will receive the information from the dendritic cells to develop a functional immune response to this threat - meaning you could potentially heal faster! Think of it as these cells meeting up to share information on any invaders to be more effective at fighting them off! Meanwhile, the inflammatory part of immunity does its best to prevent any infection from progressing further.

At the end of the day (pun intended), it seems likely that it is a combination of these factors (distraction, position, and dynamics of the immune system) that cause us to feel worse at night. Unfortunately, there still isn’t much to do to prevent this phenomenon other than what you should already be doing to treat a cold. Get well soon, Queen’s!