BTW, only 30% of students get this question right! Do you know the correct answer? 

Study participants are asked to live in a dark room for several consecutive days. Assuming healthy participants with normally functioning immune systems, their brains would be expected to release less of which hormone?

A. Melatonin

B. ACTH

C. Serotonin

D. Thyroxine

Explanation

Adrenocorticotropic hormone (ACTH) is released by the anterior pituitary gland when stimulated by corticotropin-releasing factor (CRH) from the hypothalamus. This chain of events is triggered by the increase in light incident on the retinas during waking. In a dark environment, this cascade would likely be disrupted, since the hypothalamus would receive fewer stimuli to promote the release of CRH, making B the most likely answer.

A. Melatonin; incorrect. Melatonin, a hormone often used as a sleep-enhancing supplement, is actually inhibited by bright light. Thus, a dark environment would (if anything) upregulate its release.

C. Serotonin; incorrect. Serotonin is typically considered to be a neurotransmitter, not a hormone, in the brain.

D. Thyroxine; incorrect. Thyroxine release is not related to darkness.

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