This test measures the free, bioavailable component of cortisol through a saliva sample. Cortisol is known as the stress hormone. When we are confronted with a threat, cortisol targets the central nervous system, the metabolic system, the immune system, and the cardiovascular system, with the aim of releasing energy to allow us to deal with the stressor and not waste precious resources elsewhere. Single-sample tests of cortisol may be problematic due to large variation across the day, increasing sharply in the first 30-60 minutes upon wake-up and then steadily declining through the day.