A few reasons: firstly, secretions in lungs are prone to the effects of gravity. Although there is a mechanism by which tiny hairs (cilia) sweep mucus up to the throat to be coughed or swallowed, if there is more mucus than the cilia can cope with then it builds up in the lungs. When you lie down, gravity helps the cilia move mucus up towards the throat. Secondly, night cough (especially in children) can be a sign of asthma. If the airways are slightly tight, it makes mucus movement more difficult, and as the airways relax the mucus moves more easily upwards. Thirdly, if you mean evening cough in children as opposed to when they're lying down - my toddler has a longstanding chest problem. She also gets manic towards 5pm, kind of a last burst of energy before going to bed. All that running around makes her cough as mucus is loosened by vigorous movement (eg coughing after running or laughing.) If children have persistent night cough it's worth seeing an asthma nurse (a lot of GP practices have them) - the child may not necessarily have asthma but they obviously have more secretions than they can cope with. Other causes include dust mite/pet/pollen sensitivity, and the asthma nurse can advise on reducing these triggers - poor nights' sleep equals tetchy kids!