When you feel like you can't breathe when you lay down it's known as orthopnea.
Orthopnea or feeling like you can't breathe when you lay down can be caused by conditions such as COPD, heart failure, obesity, sleep apnea, anxiety, fluid on the lungs etc.
When you lay down it can also exacerbate conditions like asthma or pulmonary edema which make it more difficult to breathe.
Common conditions that can lead to you feeling like you can't breathe when you lay down include.
Pneumonia which is infection of your lungs which can cause fluid and inflammation and make it difficult to breathe and it can be aggravated when laying down.
Pleural effusion which is when fluid builds up around your lungs and makes it difficult to breathe which can be made worse when laying down.
Diaphragm Paralysis which is weakness or paralysis of the diaphragm which is the muscle that controls breathing and can make it difficult to breathe, especially when laying down.
Sleep apnea is a sleep disorder which involves the pausing of breathing during sleep and is often due to airway obstruction and can be more pronounced when you lay down.
Pulmonary edema is when fluid builds up in your lungs and is often caused by heart failure and it makes it difficult to breathe, especially when laying down flat.
COPD or chronic obstructive pulmonary disease is a condition that causes inflammation and damage to your lungs and makes it difficult to breathe and lying down can also worsen the condition.
Heart failure is when your heart is weakened or not working properly and cannot pump blood through the body effectively which leads to fluid buildup in your lungs and makes it harder for you to breathe, especially when you lay down.
If you continue to have trouble breathing or feel like you can't breathe when laying down you should see a doctor or go to urgent care or the ER if it's an emergency.