Changes of the human core body temperature rhythm and sleep structure by 6-hour phase advance treatment under a natural light-dark cycle

Objectives: The purposes of this study were to reveal how far the core body temperature rhythm phase advances and the corresponding changes in sleep structure. The extremities of core body temperature rhythm shifts and nocturnal sleep structure changes were examined during 6 days of 6-hour phase advance treatment using bright light and melatonin under the natural light-dark cycle. Methods: Six healthy males received phase advance treatments with 1 hour bright light exposure after waking, oral melatonin (1.0 mg) administered in the early evening, and advancement of environmental routines intended to advance the onset of the sleep period 1 hour per day. Core body temperature was recorded continuously for 8 days comprising adaptation, baseline, and 6 treatment days. Nighttime sleep quality was evaluated by polysomnography (PSG) on adaptation, baseline, night 3, and night 6. Results: The core body temperature nadir in each day compared with baseline advanced significantly ( p < 0.05). The mean nadir had advanced approximately 4.5 hours from baseline by day 6. The only significant change found in sleep structure was REM sleep duration, which was significantly decreased in day 6 compared with baseline (p < 0.05). Significant negative correlations existed between nadir phase advances and %REM in baseline, day 3, and day 6 recordings. Discussion: Thus, phase advances greater than 4 hours were possible under natural light-dark condition, although a phase shift of 1 hour per day may be too rapid to maintain normal sleep structure.