Research spanning over half a century now shows our bodies actually respond differently at day and night. The reason for this is our body clock, and the fact that each cell in the body, including our immune cells, can tell what time of day 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 indicate the time depending on their levels.
When a virus infects a person (host), it invades the cells of its host in order to survive and replicate. Once inside, the cells of the immune system cannot ‘see’ the virus and therefore do not know that the host cell is infected. When microorganisms, such as bacteria or viruses infect us, our immune system jumps into action. It is highly trained to sense and eliminate infections and clear up any damage caused by them. It is typically assumed our immune systems work the exact same way regardless of whether an infection occurs during the day or at night.
Knowing whether it's day or night means our body can adjust its functions and behaviours to the correct time. Our body clock does this by generating 24-hour rhythms (also termed circadian rhythms) in how cells function. For example, our body clock ensures that we only produce melatonin as night falls, as this chemical makes us tired - signalling it's time for sleep.
Given the body clock's control over our immune system, it's hardly surprising to learn that some research has shown that the time we're infected with a virus, such as influenza or hepatitis, can impact how sick we become.