Light exposure in late night/early morning typically causes what shift in circadian rhythms?

• A. Phase delay
• B. No shift
• C. Phase advance
• D. Phase reset twice daily

Answer: (C)

Light is the main cue that resets our internal clock. When bright light occurs in the late night through the early morning, it signals that dawn is near, which shifts the circadian clock earlier in the cycle. This forward shift is called a phase advance, meaning the internal day begins sooner and you’re inclined to feel sleepy earlier and wake earlier. The effect happens because light input to the brain’s clock (the SCN) during this window moves the timing forward. In contrast, light exposure in the evening tends to push the clock later (a phase delay).