Randomized trial of polychromatic blue-enriched light for circadian phase shifting, melatonin suppression, and alerting responses

Abstract

Wavelength comparisons have indicated that circadian phase-shifting and enhancement of subjective and EEG-correlates of alertness have a higher sensitivity to short wavelength visible light. The aim of the current study was to test whether polychromatic light enriched in the blue portion of the spectrum (17,000 K) has increased efficacy for melatonin suppression, circadian phase-shifting, and alertness as compared to an equal photon density exposure to a standard white polychromatic light (4000 K). Twenty healthy participants were studied in a time-free environment for 7 days. The protocol included two baseline days followed by a 26-h constant routine (CR1) to assess initial circadian phase. Following CR1, participants were exposed to a full-field fluorescent light (1 × 10¹⁴ photons/cm²/s, 4000 K or 17,000 K, n = 10/condition) for 6.5 h during the biological night. Following an 8 h recovery sleep, a second 30-h CR was performed. Melatonin suppression was assessed from the difference during the light exposure and the corresponding clock time 24 h earlier during CR1. Phase-shifts were calculated from the clock time difference in dim light melatonin onset time (DLMO) between CR1 and CR2. Blue-enriched light caused significantly greater suppression of melatonin than standard light ((mean ± SD) 70.9 ± 19.6% and 42.8 ± 29.1%, respectively, p < 0.05). There was no significant difference in the magnitude of phase delay shifts. Blue-enriched light significantly improved subjective alertness (p < 0.05) but no differences were found for objective alertness. These data contribute to the optimization of the short wavelength-enriched spectra and intensities needed for circadian, neuroendocrine and neurobehavioral regulation.

Introduction

The human circadian pacemaker is exquisitely responsive to ocular light exposure [1]. Photic information is transduced by the retinohypothalamic tract to the suprachiasmatic nucleus (SCN) and then to the pineal gland via a multisynaptic pathway [2]. The SCN-pineal, but not the retinae-SCN pathways need to be intact for the secretion of melatonin, even in the absence of light [3,4]. By way of this neuroanatomy, cycles of light and dark which are detected through the eyes entrain SCN neural activity which, in turn, entrains the rhythmic synthesis of multiple physiological, endocrinological, metabolic and behavioral systems including melatonin secretion from the pineal gland [5].

In virtually all species including humans, high levels of melatonin are secreted during the night and low levels are secreted during the day [6]. Ocular light exposure affects melatonin secretion in two major ways –light exposure during the biological night causes the acute suppression of melatonin [7] and light exposure throughout the 24-h day shifts its rhythm according to a Phase Response Curve [8]. The strong circadian control of melatonin and its sensitivity to light timing mean that melatonin is often used as a marker of the circadian clock [9].

Light is detected primarily by a specialized subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) [[10], [11], [12], [13]]. These non-rod, non-cone photoreceptors express the photopigment melanopsin which has a short wavelength peak sensitivity around 480 nm [14,15].

Using this defined spectral sensitivity of the melanopsin photopigment in ipRGCs, multiple studies have examined the spectral sensitivity of melatonin suppression, circadian phase shifting, and the acute effects of alertness using narrowband (15 nm or greater halfpeak bandwidth) or monochromatic (<15 nm halfpeak bandwidth) light sources. Monochromatic blue light has been shown to cause greater circadian phase shifting and acute alerting responses over monochromatic green light [[16], [17], [18], [19], [20]].

Studies have also documented short-wavelength sensitivity for neuroendocrine effects using broad spectrum polychromatic blue-enriched light evoking stronger melatonin suppression than those with relatively less short-wavelength light [21,22]. In both of these studies, equal photopic lux bright light exposures of either 1000 lx [22] or 2500 lx [21] at high CCT (6480-6500 K) caused greater increases in melatonin suppression and increases core body temperature than low CCT (3000–3150 K) broad spectrum polychromatic light. Sato and colleagues reported that a 2 h light exposure in the early morning caused significant acceleration in the rise of core body temperature and fall of salivary melatonin secretion from the blue-enriched, high CCT condition [21]. Evening light exposures of 5 h were employed by Morita and Tokura causing significant suppression of the nocturnal rise urinary melatonin and the fall of core body temperature in the high CCT group [22]. These carefully done initial studies, however, were limited in that they examined only the acute neuroendocrine effects of relatively intense polychromatic light sources. We built on these results to examine both acute and circadian neuroendocrine responses as well as neurobehavioral differences using a standard light source at more common lower room illuminances.

In addition to highly controlled laboratory studies, several real-world applications in schools and offices using both static and dynamic lighting manipulations involving blue-enriched light have demonstrated increased subjective measures of alertness and affect as well as improvements in standard tests of cognitive processing speed, concentration and reading ability in blue-enriched lighting conditions [[23], [24], [25], [26], [27]]. For example, office installation of the fluorescent lamps used in this study on two floors with participants being exposed in a counter-balanced order resulted in statistically significant improvements in self-reported measures of alertness, mood, performance, evening fatigue, irritability, concentration and eye discomfort during the month of 17,000 K exposure as compared to 4000 K [23]. Mott et al. (2012) studied third grade students (age 7–8 years) under the Normal (500 lx, 3500 K) versus Focus (1000 lx, 6500 K) light conditions and found that the higher CCT lighting was reported to improve oral reading fluency when assessed over a full calendar year [25].

Recently, we conducted a within-subjects study that established full-range fluence-response curves to three types of fluorescent lamps that differed in their relative emission of light in the short wavelength end of the visible spectrum between 400 and 500 nm demonstrating that increasing corneal irradiances of light evoked progressively increasing suppression of nocturnal melatonin. Comparison of these fluence-response curves supports the hypothesis that polychromatic fluorescent light is more potent for melatonin regulation when enriched in the short wavelength spectrum [28]. Building on these data, we tested the hypothesis that blue-enriched polychromatic fluorescent light (17,000 K) can be more effective than standard white fluorescent light (4000 K) light for eliciting melatonin suppression, enhancement of neurobehavioral function and phase shifting in healthy human participants. This carefully controlled comparison of a high color temperature polychromatic light source was performed under commonly occurring room illuminances while balancing for photon density.