Short answers, pulled from the story.
A circadian rhythm is a natural internal biological oscillation that repeats roughly every 24 hours. To qualify as circadian, the rhythm must persist in constant conditions without external cues, must be resettable by stimuli such as light, and must remain stable across a range of physiological temperatures.
Franz Halberg coined the term "circadian" in 1959, deriving it from the Latin circa (about) and dies (day). The International Society for Chronobiology formally adopted the definition in 1977.
The suprachiasmatic nucleus (SCN) is a pair of distinct cell clusters in the mammalian hypothalamus that acts as the body's primary circadian clock. It receives light information from specialized melanopsin-containing ganglion cells in the retina and relays timing signals to the pineal gland, which secretes melatonin.
Jeffrey C. Hall, Michael W. Young, and Michael Rosbash received the Nobel Prize in Physiology or Medicine in 2017 for their discoveries of the molecular mechanisms controlling circadian rhythms. Their work identified and characterized the period gene and the feedback loops that drive the Drosophila biological clock.
Circadian disruption is associated with obesity, diabetes, hypertension, cardiovascular disease, depression, bipolar disorder, and increased risk of dementia in chronic night shift workers over 50. Shift workers show elevated C-reactive protein levels and higher blood pressure, and individuals with substance use disorders display disrupted circadian rhythms.
A 1999 Harvard University study estimated the natural human circadian period to be approximately 24 hours and 11 minutes. A 2010 study found that women average 24.09 hours and men 24.19 hours, with women tending to wake earlier and showing a greater preference for morning activity.