system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If you are congested (allergies/cold), how does the nasal passages become less congested during sleep?
gpt
Many of the symptoms you feel when you have allergies/cold are caused by your immune system trying to fight off the "invaders." Congestion and swelling is caused by increased blood flow to the affected area so that more immune cells can reach the area. A runny nose is the immune system telling the nose to try and flush out the invaders.

The immune system follows your body's circadian rhythm also known as your body's daily clock. At certain times of day, the immune system is more active so you feel more congestion while at other times, it's less active so you feel less congested at night.

People with rheumatoid arthritis, where your immune system starts attacking your own joints, will experience more stiffness in the early morning when the immune system is more active.

Why not always be active? Like your body and mind, your immune system also needs rest to recover its fighting force. Stress can disturb your circadian rhythm so that your immune system is constantly active. This gives less time to recover so your immune system will weaken with chronic stress. Chronic stress and disturbed rhythm can also cause your immune system to be overactive, leading to auto-immune diseases or cancer from chronic inflammation.