0026 Later Sleep-Wake/Dark-Light Times Promote Later Circadian Phase Independent of Sleep Regularity

Abstract Introduction Although irregular sleep-wake patterns are associated with delayed circadian phase, it remains unknown whether the irregular sleep-wake patterns themselves or their associated dark-light patterns are primarily responsible. Methods Twenty-seven healthy participants (9 females; ages 19-35 y) completed a 26-day inpatient study consisting of: (i) one 24-hour baseline day; (ii) 21 days on a randomized Early Regular (ER), Late Regular (LR), or Irregular (IR) sleep-wake/dark-light schedule; (iii) a 30-hour interval of 90-minute “ultradian days”; and (iv) a final recovery day. Each randomized schedule was based on observed schedules from a study in college students. Schedules were selected for regularity as defined by a Sleep Regularity Index >80 (ER and LR schedules) or < 60 (IR schedule) and for timing as either Early (ER, scheduled sleep midpoint mean±SD: 3:57±0:39) or Late (LR: 6:29±0:40; IR: 7:01±1:58). Change in circadian phase was calculated as the difference in dim light melatonin onset (DLMO) between day 1 (prior to sleep-schedule intervention) and day 23 (after sleep-schedule intervention). Results There was a significant effect of schedule on circadian phase (p=0.005); phase was more delayed following LR (-2.90±2.31 h) and IR (-1.54±4.75 h) compared to ER (0.23±1.41 h, p=0.001 and p=0.04, respectively) with no difference between LR and IR (p=0.25). There was a significant effect of schedule (p=0.0005) on light exposure in the phase advance region (measured by area under the curve [AUC] of photopic illuminance from 6 h to 15 h after DLMO): the AUC was significantly higher for ER compared to LR (p=0.007) and IR (p=0.0005) but not different between LR and IR (p=0.37). Light in the phase delay region (AUC of photopic illuminance from 3 h before to 6 h after DLMO) trended towards but did not reach significance across the three schedules (p=0.07). Conclusion LR and IR schedules had later circadian phase and less phase advancing light compared to ER schedules, suggesting that later circadian phase in late and irregular schedules is due primarily to the temporal pattern of light exposure independent of sleep regularity. Support (if any) NIH R01HL128538; NIH UL1TR002541