Cortisol is a glucocorticoid hormone. It is the most important one in humans, produced by the adrenal cortex and participates in the body’s homeostasis and stress responses. Cortisol concentrations also follow a circadian rhythm. It is a more complex rhythm than the human melatonin rhythm. Unlike the melatonin rhythm, human cortisol rhythms do not seem to be totally associated with day and night per se, but seem to be more closely tied to the “transition periods” from dark to light and to a lesser extent, from light to dark. Transitioning light levels play a tremendous role in cortisol rhythms in humans. In addition to its circadian rhythm exhibiting a predictable peak in the morning, cortisol levels typically elevate sharply in the morning, 30 minutes to an hour after awakening. The glucocorticoid levels synthesized by the adrenal gland across the 24 hour day appear to be under the control of two distinct systems, one governed by the hypothalamic-pituitary-adrenal (HPA) axis, and one controlled by the autonomic nervous system through the adrenal medulla. Evidence supports that cortisol production can be uncoupled from the HPA axis controller of its release (ACTH). Night time light stimulates the suprachiasmatic nucleus (SCN) and this sends a neural signal to the autonomic systems to increase cortisol production from the adrenal gland, but not the brain. This is not coupled to pulsatile ACTH release in the pituitary, and has separate neural pathways. Studies have shown that exposure to high levels of polychromatic (white) light (80lux at the cornea) in the morning, but not in the evening, could increase cortisol levels in humans. It appears the intensity of light is critical to the real effect on cortisol levels. Studies have also shown that morning light can increase heart rate, suggesting an impact of light on the autonomic nervous system that modulates cortisol release from the adrenal gland. More recent studies have shown bright light to dramatically reduces cortisol levels in humans.
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