Bright Light Exposure at 9 PM Hawaii Time (Answer: C)
For this westward traveler crossing 5 time zones, bright light exposure should occur at 9 PM Hawaii local time to optimally advance his circadian rhythm and accelerate adaptation to the new time zone.
Rationale Based on Circadian Phase Response Curve
The key to answering this question lies in understanding the phase response curve to light and applying it to westward travel 1. Light exposure before the core body temperature minimum (or biological night nadir) delays circadian phase, while light after this point advances circadian phase 1.
Understanding His Circadian Timing
- Home sleep schedule: 11 PM - 6:30 AM (Eastern Time)
- Circadian nadir (core body temperature minimum): Occurs approximately 2-3 hours before habitual wake time, so around 3:30-4:30 AM Eastern Time
- In Hawaii time: This translates to approximately 10:30-11:30 PM Hawaii time (5 hours earlier)
Why 9 PM Hawaii Time is Optimal
When he arrives in Hawaii at 1:53 PM local time:
- 9 PM Hawaii time = 2 AM Eastern time (his home time)
- This is before his circadian nadir in biological time
- Light exposure at this time will delay his circadian rhythm 1
This is exactly what he needs for westward travel - he needs to delay his internal clock to match the later Hawaii schedule 2.
The Westward Travel Strategy
For westward travel, circadian adaptation requires a phase delay, which is achieved through 2:
- Afternoon and early-evening light exposure at the destination
- This helps shift the sleep-wake cycle later to match local time
The guideline evidence clearly states: "For westward travel, a circadian delay can be achieved after arrival with afternoon and early-evening light" 2. The 9 PM exposure falls within this critical window while still being before his biological night nadir.
Why Other Options Are Incorrect
- 1 AM, 3 AM, or Midnight Hawaii time: These correspond to 6 AM, 8 AM, or 5 AM Eastern time respectively - all occurring after his circadian nadir. Light at these times would cause phase advances rather than the needed delays 1, working against westward adaptation.
Critical Timing Principle
The evidence emphasizes that timing relative to the phase response curve is paramount 1. Even with optimal light intensity and duration, exposure at the wrong circadian time will produce the opposite effect or no effect at all. The 9 PM Hawaii exposure strategically targets the pre-nadir delay zone while being practically feasible (he's awake and can seek bright light exposure).
This recommendation aligns with the principle that light exposure must be timed according to the traveler's biological time, not just local clock time, especially within the first 24 hours when the circadian system is still entrained to the home time zone 3.