Stress is a psychological and physiological response that occurs when an individual perceives that the demands of a situation exceed or threaten their ability to cope. In behavioural sciences, stress is understood as an interaction between a person and the environment. It is influenced not only by external events but also by how an individual interprets those events. Stress is a normal part of life and, in moderate amounts, can improve alertness, motivation and performance. However, excessive, prolonged or poorly managed stress can negatively affect physical health, mental well-being, behaviour and social relationships. Stress can be classified into different types according to its duration, source and psychological effect. One important type is acute stress. It is a short-term response to an immediate challenge or threat. Situations such as an examination, interview, accident, argument, sudden financial problem or public speaking can produce acute stress. The body quickly activates the sympathetic nervous system, preparing the individual to deal with the situation. Heart rate and breathing increase, muscles become tense and attention becomes focused. Acute stress generally disappears when the stressful situation ends. Although it can be uncomfortable, brief acute stress may help a person respond quickly and effectively to a challenge. Episodic acute stress occurs when an individual experiences acute stress repeatedly or frequently. People who constantly face deadlines, conflicts, excessive responsibilities or disorganised lifestyles may experience this form of stress. Individuals who are highly competitive, impatient, perfectionistic or constantly worried may also be more vulnerable. Repeated episodes of stress can prevent the body from returning completely to its normal state and may contribute to headaches, fatigue, irritability, sleep problems and difficulty concentrating. Chronic stress is long-lasting stress that continues for weeks, months or even years. It may arise from persistent unemployment, financial difficulties, family conflict, relationship problems, an unsafe work environment, prolonged caregiving responsibilities or continuing academic and occupational pressure. Chronic stress is particularly harmful because the body's stress response remains activated for extended periods. Over time, it can disturb normal physiological functioning and increase vulnerability to several physical and psychological problems. Another useful distinction is between eustress and distress. Eustress refers to positive or beneficial stress that motivates a person to perform better and adapt to challenges. For example, moderate pressure before an examination may encourage a student to study more effectively. Similarly, the challenge associated with starting a new job or undertaking an important project may increase energy and motivation. Eustress is generally associated with excitement, engagement and improved performance when the demands remain manageable. Distress refers to harmful or unpleasant stress that occurs when perceived demands exceed a person's coping resources. It may produce feelings of anxiety, helplessness, frustration, anger or sadness. Distress is more likely when an individual feels that a situation is uncontrollable, unpredictable or threatening. Persistent distress can interfere with everyday functioning and