Introduction: Melatonin and various patterns of bright light and dark have been tested to help phase shift the circadian clock and promote adjustment to night work and day sleep schedules. This study tested various combinations of these interventions to determine their ability to produce phase delays. Methods: Healthy young adults (32 males, 35 females, 23.9 +/- 6.2 yrs) participated in the summer months of 3 years. They worked 5 consecutive simulated night shifts (23:00 to 07:00) followed by sleep at home (08:30 to 15:30). Bedroom windows were darkened with black plastic. While traveling home, subjects wore sunglasses with normal or dark lenses (transmitted 15% or 2% of light respectively). Subjects took a placebo or melatonin (1.8 mg sustained release) pill before daytime sleep. During the night shifts, subjects were exposed to a moving pattern of intermittent bright light (approximately 5000 lux, 20 min on, 40 minutes off, 4-5 light pulse/night) or remained in dim light (approximately 200 lux). There were 6 intervention groups: 1) Dark/Sleep + Normal Sunglasses; 2) Dark/Sleep + Dark Sunglasses; 3) Dark/Sleep + Dark Sunglasses + Melatonin; 4) Dark/Sleep + Normal Sunglasses + Bright Light; 5) Dark/Sleep + Dark Sunglasses + Bright Light; 6) Dark/Sleep + Dark Sunglasses + Melatonin + Bright Light. There was a circadian phase assessment before (baseline) and after (final) the night shifts to determine the salivary dim light melatonin onset (DLMO). Sleep and night shift performance data are reported in an accompanying abstract (Lee et al). Results: The final DLMO (mean clock time (SD in h)) in groups 1 through 6 was 2:29 (4.0), 4:19 (3.0), 4:42 (1.5), 6:08 (3.1), 6:57 (0.5), and 7:31 (0.6), respectively. Thus, the three bright light groups achieved complete re-entrainment to the daytime sleep schedule (DLMOs were 1- 2.5 h before bedtime at 8:30). There was no difference among the three bright light groups and their DLMOs were significantly later than those of group 1. The difference between groups 1 and 2 (which differed only in type of sunglasses) did not reach statistical significance, showing that very dark sunglasses added little to the phase delay. There was no significant difference between groups 2 and 3 (which differed only in that one took melatonin and one took placebo), showing that melatonin does not increase the phase delay given very dark sunglasses. Conclusions: Given appropriately timed bright light during the night shift and a regular daytime dark period, neither very dark sunglasses nor melatonin had any added benefit. The minimum combination of interventions that produced complete re-entrainment to the daytime sleep and night work schedule was bright light during the night shift, normal sunglasses during the commute home and a regular dark episode for daytime sleep.
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