The most significant environmental cue that plays a role in circadian rhythms is __________

A) loud noise.
B) extreme heat.
C) hunger.
D) bright light.

Bright light, specifically blue light, is the most significant environmental cue that plays a role in circadian rhythms. The eyes contain photoreceptor cells that are sensitive to light and signal the suprachiasmatic nucleus (SCN) in the brain, which regulates the body's internal clock. Blue light, which is abundant in sunlight and electronic devices, suppresses the production of melatonin, a hormone that promotes sleep, and stimulates wakefulness. This is why exposure to blue light before bedtime can disrupt sleep and lead to circadian rhythm disorders.