A cardiologist warns that early morning walks in winter can increase the risk of heart attacks and strokes. Cold temperatures cause blood vessels to constrict, raising blood pressure and making the heart work harder, especially in the early morning when temperatures are lowest. This poses a greater risk for individuals with pre-existing heart conditions, uncontrolled hypertension, or a history of stroke. Sudden exertion after leaving a warm environment and higher pollution levels in the morning can also stress the cardiovascular system.