The reason colds are worse at night is because during the night your body has less cortisol in your blood.
Your body uses the cortisol in your blood to help fight off infections such as cold viruses, flu viruses and other viruses and infections.
So since your body has less cortisol in your blood at night your body cannot as easily fight off the cold virus so your cold is worse at night.
During the day you feel a little better from your cold because your blood has more cortisol in it which helps fight off the cold virus.
It's normal to feel worse with a cold or other virus or illness at night and then feel better the next day.
If you're having trouble sleeping at night with a cold or flu virus try taking some NyQuil which helps ease your cold and flu symptoms as well as helps you fall asleep.
I take NyQuil at night when I'm sick with a cold or flu and cannot sleep and within 20 minutes of taking some NyQuil I fall asleep.
And sleep is very important to your body helping heal itself and fight off infections and viruses.