Light exposure in late afternoon is associated with which circadian phase shift?

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

Answer: (B)

Light timing drives circadian phase shifts along the phase response curve. Exposure to light in the late afternoon sits on the part of the curve that delays the clock, pushing the internal timing later. In practical terms, bright late-day light can make you feel like your sleep-wake schedule shifts later, with later bed and wake times. Morning light, by contrast, tends to advance the phase (earlier sleep and wake times). The idea that late-afternoon light would cause a phase advance isn’t consistent with how the clock responds—the delay region is the correct mechanism for late-day exposure.