Phase-advanced circadian rhythm is characterized by earlier sleep-wake times; which intervention helps shift earlier?

• A. Evening light exposure
• B. Morning light exposure
• C. Melatonin in the morning
• D. Exercise at night

Answer: (B)

Exposure to light at the right time is the strongest cue for resetting the body's clock. To move a phase-advanced rhythm earlier, you want bright light in the morning. Morning light signals the start of the day to the brain's clock (the suprachiasmatic nucleus), helping to advance the timing of sleep and wakefulness over several days by suppressing melatonin earlier and promoting wakefulness. In contrast, evening light tends to push the clock later, delaying sleep. Melatonin in the morning would promote daytime sleepiness and isn’t used to advance the phase, and nighttime exercise can delay the circadian rhythm or have variable effects. Therefore, morning light exposure best shifts the schedule earlier.