Cortisol is a steroid hormone synthesised in the adrenal glands under control of the hypothalamus. Cortisol, amongst other roles, responds to stress, regulates blood sugar, and fights infections.  The both parts of the nervous system are involved in the response to stress: The central nervous system (CNS) responds to stress by activating the hypothalamus, while the peripheral nervous system modulates the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system (ANS).  In combination, these systems interact with the immune system contributing to the pathogenesis of stress related diseases.

Uniquely the HPA axis integrates both physical and psychosocial signals to maintain a wide range of the functions from metabolism,  to cardiovascular, immune, and reproductive systems.  Thus, downstream cortisol levels can be used to measure  the activity of the HPA axis. In combination with cortisol, the digestive enzyme salivary alpha-amylase (sAA) has emerged as a useful marker of ANS activity in stress research.

In most people, cortisol levels are highest in the morning on waking, decline throughout the day and lowest around midnight.  Most studies compare a baseline collected before and after the experiment.  Sometimes researchers use baselines on different days.  If you wanted to see if cortisol/ sAA was influenced by the intervention a baseline measure before/ after the task and after is probably ideal and you will need a control group to control for diurnal changes (cortisol will drop in most people who are not doing much over an hour or so and the control group allows you to see if the test group change differently to just the normal diurnal cycle).    

We are often asked about the cortisol and sAA reaction speeds, which are different and so post intervention measures in a study would need to be timed differently.  sAA reacts quickly and the declines quickly; cortisol peaks 20-40 minute after the stressor.  For example for a short task based study we may recommend a measure just after the task, one at 20mins, 40mins and then perhaps 60 mins.  Cortisol is slow to return to baseline, at 60mins cortisol will not have returned to baseline but sAA would.  This is one example, contact us for more information on study design for long tasks, multiple interventions or longitudinal studies. 

References and further reading

This is an old paper but a classic for cortisol response timings: 

Dickerson, S. S., & Kemeny, M. E. (2004). Acute stressors and cortisol responses: a theoretical integration and synthesis of laboratory research. Psychological Bulletin, 130(3), 355–391. https://doi.org/10.1037/0033-2909.130.3.355

Ali, N., Nater, U.M. Salivary Alpha-Amylase as a Biomarker of Stress in Behavioral Medicine. Int.J. Behav. Med. 27, 337–342 (2020). https://doi.org/10.1007/s12529-019-09843-x