Cyclopentolate

The Effect of Polarized Versus Nonpolarized Light on Melatonin Regulation in Humans^

ABSTRACT

The aim of this study was to compare the effects of polarized light versus nonpolarized light on melatonin secretion in healthy, humans (mean age, 25 years; N = 6). On separate evenings, each subject was exposed to four different light intensities (20, 40, 80 and 3200 1x) of both polarized and nonpolarized light, as well as to a control, dark exposure. Each evening experiment consisted of a 120 min dark exposure (0000-0200 h) followed by a 90 min light exposure (0200-0330 h). Subjects' pupils were dilated prior to exposures. Blood samples were drawn at the start and end of each light-exposure period and later assayed for melatonin by radioimmunoassay. When compared to control exposures, both polarized and nonpolarized light elicited significant suppression of plasma melatonin at each illuminance (P

INTRODUCTION

The human eye transduces environmental light stimuli into neurally encoded information for several distinct sensory purposes, including visual sensation, circadian regulation, pupillary reflexes and other ocular reflexes. With specific regard to the visual system and circadian system, both of these systems originate within the eye at the level of the retina, although their neural pathways and terminal destinations are very different. For normal vision in humans and other mammals, light passes through the ocular media to reach the retinal photosensory cells, rods and cones. In response to light, these cells transmit specific electrochemical impulses that are relayed, ultimately, to the visual cortex through a multisynaptic pathway.

Although it is not yet known which retinal photosensory cells) and photopigment(s) initiate the transduction of environmental light cues for the circadian system, it has been well established that the retinohypothalamic tract carries this information from the retina to the suprachiasmatic nuclei (SCN)^^ located within the hypothalamus (1,2). The SCN act as the body's principal circadian pacemaker, regulating and entraining daily rhythms of physiology and behavior (3,4). The pineal gland hormone, melatonin, is one of the endproducts regulated by the circadian system. The SCN are connected to the pineal gland via a multisynaptic pathway which includes, sequentially, the hypothalamic paraventricular nuclei, the thoracic intermediolateral cell column and the superior cervical sympathetic ganglia (3,5). By way of this neural pathway, environmental light and the endogenous circadian clock regulate daily production melatonin. Further, melatonin is often utilized as a marker for monitoring the effects of light on the circadian system.

In humans, as in most animal species examined, melatonin production is highest during the night when light exposure is minimal or significantly reduced, and lowest during the day when light exposure is greatest (6,7). Furthermore, ocular exposure to environmental light has been demonstrated to elicit acute suppression of nocturnal levels of melatonin (8,9), entrain this hormone's circadian rhythm of production (10,11) and adjust the duration of nocturnal melatonin secretion relative to ambient photoperiod (12). The light-induced melatonin suppression response has been used extensively for elucidating the ocular-dependent biochemistry, physiology and neural regulation of the pineal gland and melatonin production (3,9).

In addition to the effects of light on the overall circadian system as well as the specific regulation of melatonin secretion from the pineal gland, there have been numerous investigations into the biologic and therapeutic effects of light in humans. In particular, light therapy has been utilized successfully in the treatment of several affective disorders as well as problems associated with circadian phase-shifting as a result of jet travel and shift work (13-15). As various lighting sources have been developed for general illumination as well as for specific therapeutic applications, there is currently a debate regarding the use of polarized light versus the traditional nonpolarized light, as delineated in Clear and Mistrick (16). Some studies report that polarized light provides subjects with a more efficient and a more comfortable stimulus, which may also improve visual performance, whereas other reports found no significant differences when compared to nonpolarized lighting.

Wide-ranging species, including insects, invertebrates and vertebrates, have been demonstrated to have the ability to detect specific polarities of ambient light. For example, many of these animals can discern the direction of light polarity, utilizing it for functions such as navigation, spatial orientation, object recognition and intraspecies communication (17,18). De Vries and Haidinger (cited in De Vries et al. (19)) demonstrated that humans have the ability to visually discriminate polarized light, although they cannot detect the direction of polarity. However, it is not known if, on humans, light polarity has any specific visual or nonvisual effects that are not produced by nonpolarized light. Furthermore, all the studies that have examined the effects of light on the regulation of mammalian (including human) neuroendocrine and circadian physiology have employed nonpolarized light. The current study was undertaken to assess a possible role of polarized light in the regulation of circadian functions in humans by comparing the effects of vertically polarized light and nonpolarized light on melatonin regulation in healthy human subjects.

MATERIALS AND METHODS

Subject recruitment. A total of six subjects (four male and two female; mean age, 25 +/- 1 [standard error of mean, SEM] years) were recruited from the Thomas Jefferson University community population. Neither race nor ethnic background was considered in volunteer selection. Before being approved for participation in this study, each subject reported being free of any medication as well as having regular sleep patterns; each selected volunteer also demonstrated normal color vision when tested with an Ishihara pseudoisochromatic plate test. Volunteers signed informed consent forms, which were approved by the Institutional Review Board of Thomas Jefferson University.

Experimental design. The basic experimental protocol utilized in this study is illustrated in Fig. 1. Subjects arrived at the experimental site at approximately 2330 h. Both pupils were dilated with the mydriatic agent, 0.5% cyclopentolate HCL (Cyclogyl, Alcon Laboratories, Ft. Worth, TX), between 2345 and 0000 h. Subjects manually compressed each (bilateral) set of lacrimal canaliculi and sacs for 1 min after application of the drops in order to minimize systemic absorption of the drug as well as to maximize its local mydriatic effects. From 0000 to 0200 h the subjects were blindfolded to prevent any ocular exposure to ambient room-light. Blindfolds were removed at 0200 h, and each subject was then positioned at a fighting workstation with their head and eyes positioned approximately 30 cm from the light-emitting surface of the luminaire. The subjects were then exposed to one of eight lighting conditions during each, separate test period (vertically polarized and nonpolarized light, each at 20, 40, 80 and 3200 1x). A control, dark exposure, was also provided during a separate experimental test period in which the subjects were blindfolded for the entire 3.5 h test period. Thus, the experimental design consisted of nine individual experimental sessions-eight different lighting conditions and one control period. Test sessions were separated from one another by at least 1 week. It was not possible to randomly assign stimulus intensities to a given subject, although the selection of stimulus polarity was randomly assigned. Initially, all subjects (N = 6) were exposed to a high stimulus intensity in order to establish a saturation response (between light intensity and melatonin suppression). Once this was established (3200 1x), a significantly lower intensity (80 1x) was utilized in an attempt to establish a threshold level. The threshold value was determined to be above 80 1x. The same was found for 40 1x. A 20 Ix stimulus was considered to be very close to the threshold value. Thus, it was not possible to randomly assign stimulus intensities to a given subject.

Blood sampling and melatonin assay. Antecubital blood samples were drawn by a phlebotomist before and after each experimental light exposure-at 0200 and 0330 h, respectively. They were separated by centrifugation at 2000 g for 15 min, aliquoted into cryogenic vials and stored at -20 deg C. Melatonin content was determined by radioimmunoassay (RIA) using a technique derived from Rollag and Niswender (20). This entailed producing chloroform extracts of the plasma samples, which were washed twice with 15 vol of (3 mL) petroleum ether. Radioiodinated melatonin analog was prepared by adding 1 (mu)mol of 5-methoxytryptamine and 1 (mu)mol of tri-N-- butylamine dissolved in 10 (mu)L of dioxame to 250 (mu)Ci (0.1 nmol) of dry Bolton-Hunter reagent (New England Nuclear Corp., Boston, MA); the reaction was allowed to proceed for 10 min before electrophoretic separation of products. Assay results were not corrected for recovery (which has proven to be >95% in independent trials). The minimum detection limit of the assay was 0.5-2 pg/mL. In this assay, control samples containing 23 and 113 pg/mL melatonin gave mean interassay coefficients of variation of 8.8 and 10.5%, respectively.

Lighting design. Light sources were designed collaboratively between Philips Lighting B.V. (Eindhoven, The Netherlands), Shiftwork Systems, Inc. (Cambridge, MA) and Thomas Jefferson University. Broad-band white light was produced by four 18 W triphosphor T-8 fluorescent lamps and high-frequency electronic ballasts (20-25 kHz) (Lutron Electronics Corporation, Coopersburg, PA). The light-emitting surface (61 X 61 cm^sup 2^) was equipped with a translucent lens (approximately 60% transmission) to diffuse the light and obscure the direct view of the individual fluorescent lamps suspended within the luminaire. Each luminaire was mounted in a wooden holder which enabled it to be positioned vertically upon the work-station desktop approximately 30 cm from the subject's eyes. Light illuminances (3200, 80, 40 and 20 1x) were adjusted using Rosco neutral density filters (Rosco Cinegel, Port Chester, NY) with minor adjustments made with an electronic dimming control attached to each luminaire. The use of high-speed electronic ballasts allowed for small illuminance adjustments without causing any variations in the spectral power distribution of the emitted light between the different illuminances. To provide the polarized lighting conditions, vertical polarizing filters (Philips Lighting B.V., Eindhoven, The Netherlands) were mounted over the light-emitting surface of each luminaire. Illuminance measurements were performed with a Minolta illuminance meter (Model T-1, Minolta Camera Co., Ltd, Japan) before, and every 30 min during, the 90 min exposure period.

Data calculations and testing. Two conversions were performed on the raw melatonin data. The percent melatonin change was computed by the formula, 100 x ([0330 level - 0200 level]/0200 level), as described by Gaddy et al. (21). By this computation a negative percent change score indicates melatonin suppression while a positive percent change score reflects a rise in plasma melatonin levels. Subtraction of the control, dark-exposure percent change score from the light-exposure percent change score yielded a control-adjusted index of plasma melatonin suppression. This technique accounts for the normal individual rise or fall in plasma melatonin levels with respect to the light-induced changes. One-factor, repeated-measures analysis of variance (ANOVA) was used to test for significant differences among the control exposure and each illuminance for polarized and nonpolarized light exposure. If the ANOVA was significant, a paired two-tailed Student's t-test was performed to determine any significant differences between the percent control-adjusted indices of melatonin suppression between nonpolarized and vertically polarized light at each illuminance. A two-factor, repeated-measures ANOVA was employed to test for the main effects of polarized light, illuminance and interaction of illuminance and light polarity. If there were no overall significant differences between light polarity and melatonin suppression, a power analysis was performed in order to eliminate a possible Type-II error.

RESULTS

All pre-light-exposure plasma melatonin values were typically high; this is consistent with the preceding 2 h dark-- exposure periods as well as the late hour of the sampling times. Specifically, at 0200 h the mean (+/-SEM) melatonin value was 45.8 +/- 10.7 pg/mL for the control, dark-exposure period. On nights when volunteers were exposed to light at 20, 40, 80 and 3200 1x, the 0200 h melatonin values were: 46.5 +/- 8.2, 42.3 +/- 7.8, 39.0 +/- 7.2 and 51.5 +/- 12.8 pg/mL (polarized light) and 40.0 +/- 4.8, 54.7 +/- 8.6, 33.3 +/- 6.7 and 47.4 +/- 9.5 pg/mL (nonpolarized light), respectively. Oneway ANOVA demonstrated that there were no statistical differences between these mean melatonin values (F[5,8] = 1.66, P = 0.139).

Table 1 describes the mean melatonin percent change scores for the eight light exposures as well as the control condition. Negative scores indicate a decrease in mean melatonin during the 90 min period between 0200 and 0330 h; positive scores indicate an increase in mean melatonin. The expected rise in melatonin during the control, dark period was statistically significant (P

The data illustrated in Fig. 2 describe mean (+/-SEM) melatonin control-adjusted percent change scores. Repeatedmeasures, two-factor ANOVA showed that light intensity had a significant effect on melatonin suppression (F = 15.033, degrees of freedom [df] = 3, P

DISCUSSION

Electromagnetic radiation emanating from the sun's surface is nonpolarized, with light waves transmitted in every direction. However, as the visible wavelengths pass through the earth's atmosphere, they are scattered, reflected and refracted-resulting in the production of light with varying degrees of polarity (22,23). In fact, the celestial sky contains very specific and symmetric patterns of light polarization, which vary with the position of the sun as well as with atmospheric disturbances (18,23).

More than 50 years ago, Karl von Frisch (24) discovered that honeybees were able to detect specific patterns of sunlight polarization and employ it as a compass reference for navigation. Since then, it has been demonstrated that other animal species can detect and utilize polarized light to aid in navigation, spatial orientation, object definition and location, enhancing contrast, and intraspecies communication. These include insects (17,24), marine invertebrates (25) and several vertebrate species, including fish (23), amphibians (26) and birds (27). Unique and specialized optic and neurologic physiology for the perception of polarized light has been identified in these species.

In 1884, Haidinger (cited in De Vries et al. (19)) demonstrated that human subjects were able to see linearly polarized light when presented with this single stimulus in the absence of any interfering background light. De Vries et al. (19) reinforced these findings under similar testing conditions utilizing linearly and circularly polarized light. However, when exposed to sunlight, a mix of polarized light and nonpolarized light, humans cannot specifically differentiate polarized light from ambient nonpolarized light. Furthermore, it has not been demonstrated that humans and other mammals have any specific optic and/or neurologic adaptations to sense polarized light and utilize this information in any significant or useful manner.

Studies have clearly demonstrated the beneficial effects of nonpolarized light in light therapy for the treatment of seasonal affective disorder and nonseasonal depression (11,13,14), sleep disorders (28), as well as disorders involving circadian rhythm disruption as a result of jet travel and shift work (15,29). With the continuing development of the field of human light therapy, numerous devices have been developed to deliver light for therapeutic purposes. These include light panels, work-station lighting, architectural lighting, light visors, light masks and dawn simulators ( 13,14,30-32). Despite their therapeutic benefits, these light sources often emit bright light with abundant glare that causes visual discomfort. It would be advantageous to develop therapeutic light sources that optimize visual comfort. Documented in a recent review by Clear and Mistrick (16), artificial polarized lighting was found by some studies to enhance visual comfort and performance whereas other studies found little or no effect. Clearly, there is no consensus on the possible effects, if any, of the polarized light in humans.

To our knowledge, this is the first study that examines the specific effect of ocular exposure to polarized light on neuroendocrine regulation in humans. Comparing the effect of nonpolarized light and vertically polarized light on the regulation of melatonin production, this study demonstrates that the human eye and its related circadian pathways are sensitive to vertically polarized light. It also demonstrates a light intensity-dependent response in melatonin suppression for the polarized light stimulus similar to that found in studies with humans and other mammals in which nonpolarized light was utilized (33-35). Furthermore, the level of melatonin suppression elicited at the highest illuminance (3200 1x) appears to be at, or near, saturation for both stimuli; this is consistent with earlier studies (8,33,34).

Although no significant differences were detected between vertically polarized light and nonpolarized light on melatonin suppression, the data demonstrate a possible stimulus threshold difference at 40 1x. At this intensity the nonpolarized stimulus significantly suppressed the plasma melatonin levels (P

Since only one form of light polarity was employed in this study, it remains possible that the human eye and circadian system may be able to discriminate horizontally or circularly polarized light. In order to better address the overall question of whether or not there exist significant differences between light polarity and melatonin suppression in humans, it is suggested that future studies employ a range of different forms of light polarity (e.g. vertical, horizontal, circular, etc.), as well as a broader range of light intensities which would better enable the investigator to determine the threshold intensities for the various light polarities. In addition, only a single, short-term biological response was examined in this study: would polarized light have similar effects as nonpolarized light on phase-shifting the melatonin rhythm, general entrainment of circadian rhythms or other biological responses?

In conclusion, this study demonstrates that the human eye and circadian system are sensitive to polarized light. Although this stimulus was found to suppress the plasma melatonin levels in a dose-dependent manner, similar to that of the nonpolarized stimulus, no significant differences were found between these two light modalities on this neuroendocrine parameter.

Acknowledgements-We would like to thank our phlebotomist, Robert Glasgow. We would also like to thank Ted Baker at Shiftwork Systems for the light panels utilized in this study. This work was supported by a grant from Philips Lighting B.V., Eindhoven, The Netherlands. Valuable co-support was supplied by equipment, facilities and personnel from NIH grant ROI NS36590 to G.C.B. and NSF grant IBN 9809916 to M.D.R.

^A preliminary report on this work was presented at the meeting of the American Society for Photobiology, Washington, DC, 10-15 July 1999.

*To whom correspondence should be addressed at: Department of Neurology, Jefferson Medical College, Thomas Jefferson University, 1025 Walnut Street, Suite 310, Philadelphia, PA 19107-5083, USA. Fax: 215-923-7588; e-mail: george.brainard@mail.tju.edu

2000 American Society for Photobiology 0031-8655/00 $5.00+0.00

^^Abbreviations: ANOVA, analysis of variance; df, degrees of freedom; SEM, standard error of the mean; SCN, suprachiasmatic nuclei; RIA, radioimmunoassay.

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George C. Brainard*1, Mark D. Rollag2, John P. Hanifin1, Gerrit van den Beld3 and Britt Sanford1

1Department of Neurology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA;

2Department of Anatomy, Uniformed Services University of Health Sciences, Bethesda, MD and

3Philips Lighting B.V., Eindhoven, The Netherlands

*To whom correspondence should be addressed at: Department of Neurology, Jefferson Medical College, Thomas Jefferson University, 1025 Walnut Street, Suite 310, Philadelphia, PA 19107-5083, USA. Fax: 215-923-7588; e-mail: george.brainard@mail.tju.edu

Received 10 December 1999; accepted I March 2000

Copyright American Society of Photobiology Jun 2000
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