The reason for that is enhanced at night runny nose - is gravity Bashny.Net

At night, you almost always feels tired and weary. However, nasal congestion is really strong at night. In part this has to do with our old friend - the force of gravity. Bol, you lie often and rarely get out of bed. Being in the supine position and moving a little, you are in a state where the liquid tissue and blood to the head at a higher level than when you are standing. That is why, recovering from a cold, you feel better when climb and walk.

Source: Stephen Juan, «The odd body»

via factroom.ru