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Cortisol Awakening Response (CAR)

The cortisol awakening response (CAR) is the change in cortisol concentration that occurs in the first hour after waking from sleep. It is assessed using salivary cortisol samples immediately after waking and then at intervals over the next hour. The CAR has emerged as an important aspect of hypothalamic-pituitary-adrenocortical axis function because it is regulated differently from cortisol output over the rest of the diurnal cycle. It has also been related to stress, affective disorders, and physical health risk. 

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The cortisol awakening response (CAR) is the change in cortisol concentration that occurs over the first hour after waking from sleep. It has become a topic of intense investigation following the introduction of salivary cortisol sampling, which has allowed people to collect samples noninvasively under normal life conditions at home, instead of the contrived conditions of the laboratory. Cortisol levels are low at night but rise in the early hours before waking. After waking up, most people show a further rise, peaking 20-40 min later. This is followed by a progressive reduction of cortisol over the day.

There are several reasons why the CAR has become an important research topic. First, there is evidence that the CAR is under somewhat independent control from cortisol output over the remainder of the day. There is little association between the CAR and levels over the rest of the day or the slope of cortisol decline into the evening. Second, twin studies have documented a genetic influence over the CAR that is distinct from the heritability of daytime cortisol levels. Third, the CAR is associated with stress and health in distinctive and potentially significant ways, suggesting that it is a useful marker of hypothalamic-pituitary-adrenocortical (HPA) function.

The diurnal rhythm of cortisol output is well recognized, with high levels early in the day and a decrease across waking hours reaching a nadir in the hours before waking. The actual hours over which this rhythm is entrained depend on whether the person is active at night or during the day. The secretion of cortisol is pulsatile, and it has been estimated that each secretary burst induces an increase in salivary-free cortisol. The CAR is a result of one to four secretary bursts over the period following waking. Cortisol output is under the control of a cascade of hormones initiated in the paraventricular nucleus (PVN) of the hypothalamus. The PVN itself is under strong regulation by higher structures in the brain, notably the hippocampus, amygdala, and prefrontal cortex. Additionally, there are connections between the PVN and the suprachiasmatic nucleus (SCN), a small structure in the hypothalamus that is light-sensitive and involved in circadian regulation.

The CAR is not simply the rising section of the diurnal profile but is a response to the process of waking itself. Researchers demonstrated that the rise in cortisol after awakening was steeper than could be accounted for by the diurnal cycle on its own, and the CAR is not disturbed by repeat awakenings in the night. Interestingly, when wake time is expected, there is a rise in adrenocorticotropic hormone in the hour beforehand which does not occur when waking was not expected, suggesting a preparatory response for the activities of the day. A CAR does not take place after waking from naps in the day. The CAR, but not waking cortisol level, is sensitive to light in the environment and can be enhanced by augmented light exposure toward the end of the sleep period. By contrast, noise at night has little effect.

One of the striking features of the CAR is the wide variation in the magnitude of the response both across people and within the same individual on different days.

The CAR test may be helpful for people with the following symptoms and conditions:

  • Chronic stress
  • Chronic fatigue
  • Poor sleep, including sleep apnea
  • Migraines
  • Joint pain
  • High blood pressure
  • Autoimmune diseases
  • Depression
  • Seasonal Affective Disorder (SAD)
  • Blood sugar issues
  • Gastrointestinal diseases


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