There are two instinctive stress responses, fight or flight and general adaptation syndrome.

Fight or Flight is an essential means of survival. In threatening circumstances our bodies release hormones that help us to act rapidly and escape the dangerous situation. These include elevated heart rate and blood pressure and increased oxygen and blood to activate vital muscles. This phenomenon was first described by physiologist Walter B. Cannon, as a series of biochemical changes preparing you to deal with danger. Primitive man required bursts of energy to fight or escape from predators. Once the short-term stress has ended the body returns to a normal state.

In modern times this form of stress has become far less useful to us. We don’t experience the same threats as our primitive ancestors and short-term stress often turns into longer-term chronic stress leaving us unable to make effective and focused decisions.

General Adaptation Syndrome refers to the reactions caused by longer-term exposure to stress. This was identified by the first major researcher on stress Hans Seyle, a Canadian endocrinologist.

In studies on animals that were put under extremely stressful circumstances, including subjection to blaring light, loud noise and extreme temperatures, the animals reacted in three stages:

1. Alarm phase- where they react to the stressful situation. In the alarm phase stress hormones known as catecholamines (adrenaline and noradrenaline) and glucocorticoids (cortisol) are released. Secreted by the adrenal glands these hormones put the body in a catabolic state, where glucose, fats and amino acids are broken down to release energy. This is the fight or flight situation and the heart rate, blood pressure and oxygen levels increase to improve performance.
2. Resistance phase – resistance to the stressor increased as the animal adapted to and coped with it. The body attempts to adapt to the presenting demands, although this becomes increasingly difficult. 
3. Exhaustion phase – entered into once resistance was exhausted. The body can no longer maintain a balanced state and normal functioning. 

Rats exposed to constant stress in research conducted by Seyle presented enlarged adrenal glands, gastrointestinal ulcers and wasting away of the immune system. In further studies persistent stress caused the animals to develop diseases similar to those seen in humans, such as strokes, heart attacks, kidney disease and arthritis. 

We often hear of individuals suffering from ‘adrenal fatigue’ or ‘burnout’, reaching a stage where they can no longer withstand the stresses they are experiencing. Humans experiencing chronic stress often present health problems such as adrenal problems, weakened immune system and digestive disorders.

Seyle used the term stress to refer to the body's reaction to a situation and 'stressor' to refer to the stimulus. 

Hans Seyle also differentiated between good stress and bad stress. Good stress protects us in times of danger and enables us to meet life’s challenges. Bad stress is when a person constantly experiences stress, leading the temporary fight or flight responses to occur more frequently or even continuously. This can damage our health and cause illness.