The most common type of stroke happens when a blood clot or thrombus blocks the blood flow to parts of the brain. A thrombus may form in an artery affected by atherosclerosis (the buildup of plaque in the inner wall of an artery). Plaque is made of fat, cholesterol, fibrin (a clotting material), and calcium. As plaque builds up in the arteries, blood flows slower and less smoothly, which may lead to clotting. A vessel narrowed by atherosclerosis is more likely to be blocked by a blood clot, stopping the blood flow. Thrombotic strokes usually happen at night or in the early morning. A transient ischemic attack (TIA), or "mini stroke," usually happens before a thrombotic stroke.