system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does it seem when we get sick with a cold or flu it always seems to happen overnight. For example, we always wake up with a stuffy nose or wake up with a sore throat or just wake up feeling unwell when we're coming down with something.
gpt
Hi! The infection itself happens a several days before you feel the first symptoms (so not in the night before you get sick). The cold and the flu are caused by different types of viruses that are mainly spread via aerosols and bodyfluids. After the infection, the virus needs some time to replicate in your body. That can take approximately 3-21 days. The onset of the symptoms can be at any time, but as you described, many people feel the first symptoms in the morning. And that's mainly because of the stresshormone "Cortisol" in your body. It has a lot of different functions. One of them for example is to suppress (-> "decrease") your immunsystem activity and inflammatory processes.
The amount of the secreted Cortisol is timedependand. The highest level of Cortisol in the blood can be measured around 8 o clock in the morning, the lowest concentrations can be measured in the evening. So that' s the main reason why symptoms feel worse in the morning than during the rest of the day.