0237 School-Day Phase Advancing Morning Bright Light Exposure is Limited in Teens: Support for Late School Start Times and Permanent Standard Time

Abstract Introduction It is assumed that adolescents are exposed to sunlight during their morning commute to school, a time of day when bright light can phase advance circadian rhythms (make them earlier). We calculated the duration of bright light that adolescents in Chicago, IL (41.88°N, 87.63°W) received before school, and how school start times and the biannual time change between standard time (ST) and daylight savings time (DST) impacted this duration. Methods 72 (41F) adolescents (14.3-18.0 years) wore light medallions (Actiwatch Spectrums) at chest-level for 11 school days. The average duration of bright light exposure (>1000 photopic lux) from wake-up time to 15 minutes after school start time was calculated for each participant. Mornings were excluded if the participant did not wear their medallion >50% of their commute or went to school late, leaving 3-11 mornings per participant (Median=8 days). Those with early (< 08:00, n=25) and late (≥08:00, n=47) school start times were compared using unpaired t-tests. For 8 participants, the biannual time change occurred midway through the study. Average school-day morning bright light duration during DST and ST were compared using paired t-tests. Results are reported as Mean±SD. Results The average duration of morning bright light exposure was 7.4±8.0 minutes. Those with later school start times received bright light for a longer duration than those with earlier school start times (9.0±8.6 vs. 4.2±5.7 minutes; p=0.006). The groups did not differ in commute time (p=0.360) which ranged from 4-100 minutes, or sunrise time (p=0.286), which ranged from 05:32-07:18. Participants were exposed to bright light for over twice as long during ST compared to DST (11.3±6.6 vs. 4.9±6.8 minutes; p=0.00003). Conclusion Adolescents living in Chicago were exposed to less than 10 minutes of bright light in the morning before school. The duration was halved when school started before 08:00 and when the clock time relative to sunrise was delayed during DST compared to ST. These findings support later school start times and permanent ST to maximize phase advancing morning light which could advance the fall asleep times of adolescents and increase their inadequate sleep durations. Support (if any) R01HL112756 (Crowley)