Daily cycles in the body's stress hormone system

Daily variations in adrenal hormone release are crucial for understanding the circadian timing system.

• The first evidence of a daily hormone rhythm was demonstrated in 1943 through urine analysis of 7 healthy males.
• Circadian rhythmicity was shown to be maintained in vitro by the adrenal gland shortly after its initial discovery.
• Experimental manipulation of corticosterone rhythms in the 1970s helped identify light- and food-entrainable oscillators.
• The hypothalamo-pituitary-adrenal (HPA) axis plays a significant role in conveying circadian signals from the central pacemaker in the brain.
• Adrenal glucocorticoids are important for resetting the circadian system following phase shifts.

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