What aspect of circadian rhythms might explain why we tend to feel so tired at night?

A) body temperature bottoms out on Monday mornings
B) the three biorhythm patterns converge at low points on Mondays
C) heart rates typically increase at night
D) there is an organ in the brain sensitive to light changes, which then affects our sleepiness

The organ referred to here is the suprachiasmatic nucleus (SCN), which is sensitive to changes in light and helps regulate our sleep-wake cycle. When it gets dark outside, the SCN signals to the body to start producing the hormone melatonin, which makes us feel more tired and ready for sleep. Therefore, the fact that it gets dark at night and our SCN signals for the production of melatonin is the aspect of circadian rhythms that explains why we tend to feel so tired at night.