Excess stress can manifest itself in a variety of emotional, behavioral, and even physical symptoms, and the symptoms of stress vary enormously among different individuals. Common somatic (physical) symptoms often reported by those experiencing excess stress include sleep disturbances or changes in sleeping habits, muscle tension, muscle aches, headache, gastrointestinal problems, and fatigue. Symptoms of many preexisting medical conditions can also worsen during times of stress. Emotional and behavioral symptoms that can accompany excess stress include nervousness, anxiety, changes in eating habits including overeating or undereating, loss of enthusiasm or energy, and mood changes, like irritability and depression. Of course, none of these signs or symptoms means for certain that there is an elevated stress level since all of these symptoms can be caused by other medical and/or psychological conditions.

It is also known that people under stress have a greater tendency to engage in unhealthy behaviors, such as excessive use or abuse of alcohol and drugs, cigarette smoking, and making poor exercise and nutritional choices, than their less-stressed counterparts. These unhealthy behaviors can further increase the severity of symptoms related to stress, often leading to a "vicious cycle" of symptoms and unhealthy behaviors.