Stage 1: Recognition of environmental demand

Every event in the environment, from the weather to the ringing telephone, has some sort of impact on us. Some of these events are predictable. For instance, the rent/mortgage payment will be due on the first of the month. You'll be expected to make small talk if you go to a party. Others are entirely unpredictable. It is hard to know when the baby will suddenly wake up sick and can't go to daycare, when another driver will cut you off in traffic, or when you will spill coffee on your new pants. Regardless of whether we can predict an event or not, the instant we become aware of that event taking place, we have recognized a demand.

Stage 2: Appraisal of the demand

Understanding that a demand has occurred does not automatically cause us to experience stress. In over 30 years of research, psychologists Richard Lazarus and Susan Folkman found that it is our lightening fast, and largely unconscious and automatic appraisal or judgment of our ability to meet the demand that determines just how stressful we will experience it to be. The appraisal process partially explains why a particular event may be negatively stressful to one person but not to another.

We appraise a demand by asking ourselves two questions: 1) Does this event present a threat to me? and 2) Do I have the resources to cope with this event? If we come to believe that the event is a threat to our well-being, or if we come to believe that we lack the means to effectively respond to the event, we then subsequently feel stressed. We will return to a more detailed discussion of the appraisal stage in a later section of this document.

Stage 3: Mobilization of the nervous system

To understand what happens at this stage, you need to know a little bit about the functioning of the human nervous system. The autonomic nervous system, or ANS, controls all of the automatic functions in our body. For example, your heartbeat, our body temperature, rate of breathing and digestion are all regulated by the ANS.

If we appraise an event as threatening, one branch of the ANS called the sympathetic nervous system (SyNS) automatically signals our body to prepare for action. During this mobilization phase, the SyNS prepares us for fighting or fleeing (two primary biologically driven and useful means of reacting to a physical threat) by triggering or activating the hypothalamic-pituitary-adrenal axis, or HPA axis (sometimes called the brain's 'stress circuit' ). The HPA axis involves a complex set of interactions between multiple parts of the brain and nervous system, including the hypothalamus, the pituitary gland, and the adrenal glands. This system controls the body's reactions to stress, and also handles a few other vital functions such as regulating digestion, the immune system, mood, sexual behavior, and the body's overall energy usage.

In response to a stressor, the hypothalamus (which is a centrally located part of the brain that sits above the brain stem, but below the cortex) releases corticotropin-releasing hormone (CRH). In turn, CRH acts on the pituitary gland, triggering the release of another hormone called adrenocorticotropin (ACTH) into the bloodstream. Next, ACTH triggers the adrenal glands (which are situated above the kidneys), to release the hormones cortisol and cortisone as well as epinephrine (otherwise known as adrenaline) and norepinephrine (otherwise known as noradrenaline). Both epinephrine and norepinepherine are neurotransmitters or chemical messengers that serve the brain and nervous system. Hormones are also chemical messengers, but they work primarily within the blood stream, rather than inside the brain.

The presence of cortisol works to immediately increase the amount of energy the body has available by raising glucose levels in the bloodstream. Glucose is a variety of sugar which is the body's primary fuel. Cortisol also increases levels of glucose within the brain, which helps to sharpen our attention and quicken our thinking process (just like stepping on the gas in a car causes more fuel to go into the engine, causing it to produce more power). At the same time cortisol dumps fuel into the body, it also functions to shut down body systems which are not immediately important for handling a physical threat, such as digestion, reproduction, and growth. This mobilizing effect of cortisol is generally temporary in nature, because in addition to everything else it does, cortisol tells the hypothalamus to gradually slow down production of CRH.

Similar to cortisol, elevated levels of epinephrine and norepinephrin increase your heart rate, elevate your blood pressure, speed up your reaction time, and boost your energy level. Under the combined effects of cortisol, epinephrine, and norepinephrine, the body diverts blood away from digestion and towards the muscles and the brain (to enhance physical functioning); increases oxygen levels in the blood (for an energy boost); increases the rate of perspiration (to help cool us down); releases blood clotting chemicals into the blood stream (in case of injury); and dilates the pupils (to help us see better in the dark). At the same time that cortisol and epinephrine exert their effects, both the pituitary gland (see below) and the brain are also busy releasing chemicals called endorphins and enkephalins which help relieve pain and enhance a sense of well-being.

Stage 4: Response to the threat

Once your body has been prepared for action by the various hormones and neurotransmitters described in Stage 3 (above), you are ready to respond to the stressor by taking physical action. Physiologists call what happens next the "fight-or-flight" response to highlight the two most common forms that this physical response tends to take. When we fight, we try to influence or neutralize the source of stress by striking out at it. Alternatively, we can flee and reduce our stress by escaping from the place where the stress is occurring, leaving the fighting for another day. Psychologists who conduct research on stress often add a third response possibility to the classic fight and flight options. Sometimes, rather than fighting or fleeing, we simply freeze instead. In many sports, this response is called "choking."

The fight-or-flight response is automatic and fast, which was helpful to our ancestors because it provided them with automatic responses to threats when they didn't have time to think logically about how best to handle a situation. Remember that herd of charging buffalo? Spending a long time debating the dangers of and potential responses to such a situation would probably be fatal. When faced with such an intensely physical threat, either fighting or fleeing as quickly as possible made the most sense in terms of survival.

The fight or flight response is optimized for responding to physical threats. It isn't very useful with the sort of intangible threats that are most common in today's world. It is never appropriate to punch your boss in the face, for instance, no matter how many times he piles work on you, or passes you over for a raise. Fleeing your workplace won't necessarily help you either, as you still need to get a paycheck!

Stage 5: Return to baseline

Once a stressor has been neutralized (or has been avoided successfully), the parasympathetic nervous system (PaNS; the other branch of the ANS besides the SyNS), starts to undo the stress response by sending out new signals telling your body to calm down. The PaNS slows your heartbeat and breathing, causes your muscles to relax, and gets your digestive juices flowing again. The PaNS system is designed to promote growth, energy storage and other processes important for long-term survival.