When you are getting closer to an important deadline or an exam, your heart starts running faster and you might feel hot and even begin to sweat. It is all about an environmental condition, stimulus or threat, called 'stressor'. When the brain detects a stressor, a physiological response is initiated, including a stress reaction (like the fast heart beat), a period of recovery and long-term adaptive responses, such as the enforcement of memories of stressful experiences.

Any stress reaction involves two sets of hormones: catecholamines and glucocorticoids. Catecholamines, like adrenaline and noradrenaline, work through the nervous system within seconds, a reaction called 'flight-or-fight'. Glucocorticoids, on the other hand, act through hormonal pathways, which run much slower, over the course of minutes and hours.

Cortisol is a glucocorticoid that can give a good estimate of the dynamics of stress reactions. The ways in which cortisol can be analyzed, however, differ greatly. Until a few years ago, studies were merely based upon blood, saliva and urine cortisol levels. These 'traditional' measurements provide valuable information about instant stress reactions, but have some disadvantages. For instance, daily cortisol variations and acute stress by nervousness can influence cortisol production. In addition, cortisol is secreted in pulses during our natural body rhythms, which can result in inaccurate average cortisol measurements. These and other factors make it difficult to assess the long-term effects of exposure to stress.

A new method is currently used to measure cortisol levels more accurately: the extraction of cortisol from hair. Unlike previous methods, this approach doesn't take a snapshot of cortisol levels, but quite the reverse: as hair grows, researchers can study cortisol levels (and thus stressful events) over long periods, months to years.

Looking at levels of hair cortisol offers possibilities to study the relationship between, say, stress and obesity. It has been known for a long time that people who work in shifts often have increased blood pressure as well as abdominal obesity, which is a typical effect of cortisol on tissues.

Medical specialist and researcher Dr Liesbeth van Rossum of the Erasmus Medical Center in Rotterdam, the Netherlands, used the hair cortisol approach to investigate this question. She explains "[Our] research indeed shows that shift work is associated with increased levels of hair cortisol. The disrupted sleeping pattern probably causes the cortisol's [daily body] rhythm to alter; the cumulative production of cortisol in the body is thereby apparently higher in the long-term."