Administering ________ at prescribed times during the day has been shown to enhance shifts of circadian rhythms in individuals with delayed sleep phase type, jet lag type, and shift work type circadian rhythm disorder.

A) acetylcholine
B) endorphin
C) melatonin
D) serotonin

Melatonin is a hormone naturally released by the body to regulate the sleep-wake cycle. Administering melatonin at prescribed times during the day can help shift the circadian rhythms in individuals with delayed sleep phase type, jet lag type, and shift work type circadian rhythm disorder. Acetylcholine, endorphin, and serotonin are not typically used to regulate circadian rhythms.