You may notice that your pet is not sleeping well at night, and appearing restless or even agitated. This may occur in dogs with severe heart disease as a result of fluid building up in or around the lungs (a condition known as Congestive Heart Failure). When these pets lie down (for example, to sleep) the fluid pools and accumulates, making breathing difficult. This can cause dogs to sleep on their chest instead of on their side or back, they may no longer sleep under the covers, or they may wake up in the middle of the night or seek out fresh air near a window. Some dogs will prefer to sit or stand; you might even observe them trying to fall asleep in these positions, and close observation of the breathing effort will often show abnormal breathing in these animals.