Why do I experience chest pain when lying on my left side?

When you lie on your left side, the way your body rests can put pressure on your ribs and the tissues connecting them to your breastbone. If these areas are already sore or inflamed—as happens in costochondritis—it can make the pain seem worse. Costochondritis is when the cartilage that connects your ribs to the breastbone becomes irritated, and lying on that side can press on it, causing pain. Sometimes, muscles in your chest can also get strained or tight from pressure or movement during sleep, which can add to the pain. This type of chest pain is usually not related to the heart. However, if the pain feels very strong, lasts a long time, or is accompanied by other symptoms like trouble breathing, it's important to tell a doctor, just to be safe. Your doctor can ask questions about the pain and may do tests to see if it's from the chest wall or something else.