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

Abstract

Background

Previous studies have shown that the human circadian system is maximally sensitive to short-wavelength (blue) light. Whether this sensitivity can be utilized to increase the size of phase shifts using light boxes and protocols designed for practical settings is not known. We assessed whether bright polychromatic lamps enriched in the short-wavelength portion of the visible light spectrum could produce larger phase advances than standard bright white lamps.

Methods

Twenty-two healthy young adults received either a bright white or bright blue-enriched 2-h phase advancing light pulse upon awakening on each of four treatment days. On the first treatment day the light pulse began 8 h after the dim light melatonin onset (DLMO), on average about 2 h before baseline wake time. On each subsequent day, light treatment began 1 h earlier than the previous day, and the sleep schedule was also advanced.

Results

Phase advances of the DLMO for the blue-enriched (92 ± 78 min, n = 12) and white groups (76 ± 45 min, n = 10) were not significantly different.

Conclusion

Bright blue-enriched polychromatic light is no more effective than standard bright light therapy for phase advancing circadian rhythms at commonly used therapeutic light levels.

Introduction

Scheduled exposure to light and darkness are effective tools for phase shifting the human circadian clock [1], [2]. Timed exposure to bright light is recognized by the American Academy of Sleep Medicine as an effective treatment for circadian rhythm sleep disorders, such as shift work disorder (SWD) and delayed sleep phase disorder (DSPD) [3], [4], [5]. Despite this recommendation, much research on the efficiency of light treatment remains to be done, to establish optimal parameters for the dosage (intensity and duration), timing and wavelength of light treatment in practical schedules that could be used in the home or workplace.

Evidence has recently emerged showing that circadian phase shifts in humans are most sensitive to short-wavelength light [6], [7], [8], [9], [10]. Although rod and cone photoreceptors contribute to non-image-forming behaviors such as phase shifting in animal models [11], [12], [13], and may also do so in humans [14], [15], [16], these responses appear primarily driven by a small population of intrinsically photosensitive retinal ganglion cells [17], [18], [19] containing the photopigment melanopsin [20], [21], [22], [23], [24]. The spectral sensitivity of non-image-forming (NIF) responses (e.g., the pupillary light reflex; light-induced melatonin suppression and circadian phase shifting) in humans was not known until 2001, when melatonin suppression was shown to be most sensitive to short-wavelength light [25], [26]. Consequently, most of the earlier circadian phase shifting studies using polychromatic light did not measure or did not report the amount of energy specifically in the blue portion of the visible light spectrum, but instead reported the illuminance of the light source, which typically ranged from 2000 to 12,000 lux [27], [28], [29], [30], [31], [32], [33]. Some specified the type of fluorescent lamp (e.g., “cool white” or “full spectrum”) or provided the correlated color temperature (CCT) [in ° kelvin (K)] of the lamps, which is a metric describing the relative proportion of warm versus cool colors in a light source. Most earlier studies used lamps with a CCT <7000 K because lamps with a higher CCT (containing more short-wavelength energy) were not readily available until recently. In the current study we used fluorescent lamps enriched with blue light, rated by the manufacturer as having a CCT of 17,000 K.

The spectral sensitivity of circadian phase shifting in humans has been assessed in carefully controlled studies that have used relatively dim, narrow bandwidth light administered with a specialized light delivery apparatus [6], [7], [8], [9], [10]. For example, one study pharmacologically dilated subject’s pupils, had subjects wear blackout goggles for 90 min prior to light exposure, and then administered a 6.5 h light pulse while subject’s heads were immobilized in a Ganzfeld dome [6]. Controls such as these are important for determining spectral sensitivity, but leave open the question of whether this sensitivity could be utilized in practical protocols to shift circadian rhythms, such as advancing rhythms before flying east to attenuate jet lag, or to treat a patient with DSPD. In addition, it is not known whether this sensitivity could be harnessed to increase the size of the phase shift relative to treatment with standard bright “white” light.

Lamps and light-producing devices emitting exclusively or relatively more short-wavelength energy are now commercially available [34]. This provides clinicians and patient/consumers with a variety of choices when selecting a device for light treatment, but there remains little evidence from well controlled studies demonstrating the efficacy of those devices for circadian phase shifting.

The goal of the current study was to determine whether bright blue-enriched light could phase advance the circadian clock more than standard bright white light at light levels that are currently being used for therapeutic applications, and using light boxes designed for practical applications.