Abstract

Key points Misalignment between the internal circadian clock driving daily rhythms in physiology and behaviour, such as sleepiness, performance and metabolism, and the sleepwake schedule, as occurs in jet lag and night shift work, can have profound, harmful consequences for health, performance and safety. Light applied at specific times of day can be used to shift the timing of the clock and reduce this circadian misalignment. We show for the first time that a small, commercially available, portable blue light device is capable of shifting the clock when it is administered daily over a 2 h window (90 min blue light as 30 min pulses with 15 min breaks). The direction and amount that the clock is shifted depends on the time of day that the light is administered. The results of this work provide a practical, effective light treatment that can be used in the real world. Abstract Light shifts the timing of the circadian clock according to a phase response curve (PRC). To date, all human light PRCs have been to long durations of bright white light. However, melanopsin, the primary photopigment for the circadian system, is most sensitive to short wavelength blue light. Therefore, to optimise light treatment it is important to generate a blue light PRC. We used a small, commercially available blue LED light box, screen size 11.2 x 6.6 cm at similar to 50 cm, similar to 200 mu W cm-2, similar to 185 lux. Subjects participated in two 5 day laboratory sessions 1 week apart. Each session consisted of circadian phase assessments to obtain melatonin profiles before and after 3 days of free-running through an ultradian lightdark cycle (2.5 h wake in dim light, 1.5 h sleep in the dark), forced desynchrony protocol. During one session subjects received intermittent blue light (three 30 min pulses over 2 h) once a day for the 3 days of free-running, and in the other session (control) they remained in dim room light, counterbalanced. The time of blue light was varied among subjects to cover the entire 24 h day. For each individual, the phase shift to blue light was corrected for the free-run determined during the control session. The blue light PRC had a broad advance region starting in the morning and extending through the afternoon. The delay region started a few hours before bedtime and extended through the night. This is the first PRC to be constructed to blue light and to a stimulus that could be used in the real world.