Phase delaying the human circadian clock with blue-enriched polychromatic light

Abstract

The human circadian system is maximally sensitive to short-wavelength (blue) light. In a previous study we found no difference between the magnitude of phase advances produced by bright white versus bright blue-enriched light using light boxes in a practical protocol that could be used in the real world. Since the spectral sensitivity of the circadian system may vary with a circadian rhythm, we tested whether the results of our recent phase-advancing study hold true for phase delays. In a within-subjects counterbalanced design, this study tested whether bright blue-enriched polychromatic light (17000 K, 4000 lux) could produce larger phase delays than bright white light (4100 K, 5000 lux) of equal photon density (4.2x10(15) photons/cm(2)/sec). Healthy young subjects (n = 13) received a 2 h phase delaying light pulse before bedtime combined with a gradually delaying sleep/dark schedule on each of 4 consecutive treatment days. On the first treatment day the light pulse began 3 h after the dim light melatonin onset (DLMO). An 8 h sleep episode began at the end of the light pulse. Light treatment and the sleep schedule were delayed 2 h on each subsequent treatment day. A circadian phase assessment was conducted before and after the series of light treatment days to determine the time of the DLMO and DLMOff. Phase delays in the blue-enriched and white conditions were not significantly different (DLMO: -4.45+/-2.02 versus -4.48+/-1.97 h; DLMOff: -3.90+/-1.97 versus -4.35+/-2.39 h, respectively). These results indicate that at light levels commonly used for circadian phase shifting, blue-enriched polychromatic light is no more effective than the white polychromatic lamps of a lower correlated color temperature (CCT) for phase delaying the circadian clock.

Figure 1

Diagram illustrating the study protocol. This schedule is an example for a subject sleeping 00:00–08:00 h during the baseline. On days 22–25, the shaded black area indicates when the subject was in bed in the dark in a laboratory bedroom. “S” during baseline indicates the hour during which the subject was required to go outside for at least 10 min of light exposure. Boxes with “L” indicate the 2 h when the subject sat in front of the light box in the laboratory. Grey areas on days 22–25 indicate times when the subject was required to be in the laboratory (< 60 lux). Upward arrows indicate a typical time of the DLMO. Downward arrows indicate the time of the DLMOff.

Figure 2

Phase delay of the DLMO in the blue-enriched and white light conditions. Lines connect the phase shift for the same subject in each condition.

Figure 3

Light exposure history for the three weeks preceding treatment versus phase delay of the DLMO.