Stress is a multifaceted response to events perceived as challenging or threatening, highlighting physical, emotional, cognitive, and behavioral reactions. Physically, stress can lead to fatigue, sleep disruptions, and various health issues such as frequent colds, chest pains, and nausea. Emotionally, it can manifest as anxiety, depression, irritability, and anger triggered by both minor and major life events. Cognitively, it may result in difficulty in concentration, memory, and decision-making, and individuals often lose their sense of humor. Behaviorally, it can lead to overeating or increased emotional reactivity, such as crying. A particularly severe stressor, known as a traumatic event, can have long-term psychological and health consequences.

Stress can be categorized into two types: distress and eustress. Distress arises from unpleasant stressors, such as job loss and relationship conflicts. At the same time, eustress stems from positive events that require adaptation, such as marriage, a job promotion, or the birth of a child. Even though these events are often seen as positive, they can still cause stress due to the significant lifestyle changes they demand. For example, while the birth of a baby is a joyous event, it can also lead to concerns about the child's health, contributing to stress. Research indicates that women who are highly anxious or experience negative life events during pregnancy have an increased risk of premature delivery compared to those with lower stress levels.

Researchers approach stress in three complementary ways:

1. Stress as stimulus: This focuses on identifying external stressors, such as life events like job loss or illness, that cause stress.
2. Stress as a response: This examines the psychological and physical reactions individuals have to stress, including emotional and physiological responses like anxiety or elevated heart rate.
3. Stress as a transaction: This approach looks at the dynamic interaction between stressful events and how individuals perceive, evaluate, and cope with them, emphasizing personal interpretation and coping mechanisms.

The study of stress often involves examining the outcomes of exposure to stressful stimuli, whether in laboratory settings or real-world scenarios. Scientists measure variables such as emotional responses (e.g., depression and hostility) and physiological markers, including elevated heart rates and the release of corticosteroids. Interestingly, highly stressful events, such as natural disasters, can sometimes lead to posttraumatic growth, where individuals report personal growth and positive change following adversity. Stress responses are highly individual, with people reacting differently to the same life events depending on how they interpret and cope with them.