0027 No Evidence for Sex Differences in the Human Phase Response Curve to a 6.5-Hour Light Exposure

Abstract Introduction It has been reported that women have a shorter circadian period, larger phase angle of entrainment, and more light-exposure induced melatonin suppression than men. Whether these sex differences extend to photic resetting of the central circadian pacemaker remains unknown. We examined whether the properties of phase response curves (PRCs) to light exposure differed between men and women. Methods Twenty-six adults (12 female), aged 19–33 years, completed an inpatient protocol consisting of: 1) an initial circadian phase assessment during a constant routine (CR), involving continuous wakefulness in a semi-recumbent posture under dim light (< 3 lux) with hourly isocaloric snacks; 2) a 6.5-hour light exposure centered within a 16-hour intervention day; and 3) a final circadian phase assessment during a subsequent CR. The timing of the 6.5-hour exposure was randomized systematically between participants so that exposures were distributed across the 24-hour circadian cycle. Data were pooled from two studies with near-identical inpatient protocols that implemented either a 6.5-hour monochromatic short-wavelength light exposure (480 nm, n=16, 2.8×1013 photons/cm2/s, 11.2 lux) or a 6.5-hour polychromatic light exposure (4100K, n=10, 2.6×1015 photons/cm2/s, 3,200 lux). Dim light melatonin onset (DLMO) was calculated by interpolating the time at which plasma melatonin reached 10 pg/mL. Phase shifts were calculated as the difference in timing of DLMO between CR1 and CR2. PRCs were generated by fitting a single-harmonic regression to phase shifts expressed as a function of the onset of light exposure relative to CR1 DLMO. Results Both men and women exhibited significant PRCs, with typically-timed phase delays and advances when light exposure occurred in the early biological night or early biological morning, respectively. There were no significant sex differences in the amplitude (men: 2.0, 95% CI [1.7, 2.5]; women: 2.1, 95% CI [1.6, 2.3] hours) or the crossover point (men: 6.3, 95% CI [5.6, 7.0]; women: 7.3, 95% CI [6.6, 8.0] hours after DLMO) of the PRCs, including after correction for the expected sex-specific drift in circadian phase (11 and 05 mins /day for men and women, respectively). Conclusion There was no evidence for sex differences in resetting of the central pacemaker by light. Support (if any) R01HL159207