A new approach to understanding the impact of circadian disruption on human health

Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union Street, Troy, NY 12180, USA. .
Journal of Circadian Rhythms 02/2008; 6:7. DOI: 10.1186/1740-3391-6-7
Source: PubMed


Light and dark patterns are the major synchronizer of circadian rhythms to the 24-hour solar day. Disruption of circadian rhythms has been associated with a variety of maladies. Ecological studies of human exposures to light are virtually nonexistent, however, making it difficult to determine if, in fact, light-induced circadian disruption directly affects human health.
A newly developed field measurement device recorded circadian light exposures and activity from day-shift and rotating-shift nurses. Circadian disruption defined in terms of behavioral entrainment was quantified for these two groups using phasor analyses of the circular cross-correlations between light exposure and activity. Circadian disruption also was determined for rats subjected to a consistent 12-hour light/12-hour dark pattern (12L:12D) and ones subjected to a "jet-lagged" schedule.
Day-shift nurses and rats exposed to the consistent light-dark pattern exhibited pronounced similarities in their circular cross-correlation functions and 24-hour phasor representations except for an approximate 12-hour phase difference between species. The phase difference reflects the diurnal versus nocturnal behavior of humans versus rodents. Phase differences within species likely reflect chronotype differences among individuals. Rotating-shift nurses and rats subjected to the "jet-lagged" schedule exhibited significant reductions in phasor magnitudes compared to the day-shift nurses and the 12L:12D rats. The reductions in the 24-hour phasor magnitudes indicate a loss of behavioral entrainment compared to the nurses and the rats with regular light-dark exposure patterns.
This paper provides a quantitative foundation for systematically studying the impact of light-induced circadian disruption in humans and in animal models. Ecological light and activity data are needed to develop the essential insights into circadian entrainment/disruption actually experienced by modern people. These data can now be obtained and analyzed to reveal the interrelationship between actual light exposures and markers of circadian rhythm such as rest-activity patterns, core body temperature, and melatonin synthesis. Moreover, it should now be possible to bridge ecological studies of circadian disruption in humans to parametric studies of the relationships between circadian disruption and health outcomes using animal models.

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    • "This sampling rate was chosen to maximize the recording time of the Daysimeter-Ds, which are limited by onboard memory capacity. The amount of 24-h behavioral alignment with work schedules exhibited by subjects during the study was quantified by performing phasor analysis on the light-dark and activity-rest patterns recorded by the Daysimeter-D (Rea et al., 2008). Phasor magnitude represents the strength of association between the 24-h activity-rest pattern and the 24-h light-dark pattern; greater phasor magnitudes suggest higher levels of circadian entrainment, although, strictly speaking, phasor magnitudes for the present study characterize behavioral alignment with the light-dark cycle (Rea et al., 2010), not necessarily entrainment of the central oscillator. "
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    ABSTRACT: United States Navy submariners have historically lived with circadian disruption while at sea due to 18-h-based watchschedules. Previous research demonstrated that circadian entrainment improved with 24-h-based watchschedules. Twenty-nine male crew members participated in the study, which took place on an actual submarine patrol. The crew were exposed, first, to experimental high correlated color temperature (CCT = 13,500 K) fluorescent light sources and then to standard-issue fluorescent light sources (CCT = 4100 K). A variety of outcome measures were employed to determine if higher levels of circadian-effective light during on-watch times would further promote behavioral alignment to 24-h-based watchschedules. The high CCT light source produced significantly higher circadian light exposures than the low CCT light source, which was associated with significantly greater 24-h behavioral alignment with work schedules using phasor analysis, greater levels of sleep efficiency measured with wrist actigraphy, lower levels of subjective sleepiness measured with the Karolinska Sleepiness Scale, and higher nighttime melatonin concentrations measured by morning urinary 6-sulfatoxymelatonin/creatinine ratios. Unlike these diverse outcome measures, performance scores were significantly worse under the high CCT light source than under the low CCT light source, due to practice effects. As hypothesized, with the exception of the performance scores, all of the data converge to suggest that high CCT light sources, combined with 24-h watchschedules, promote better behavioral alignment with work schedules and greater sleep quality on submarines. Since the order and the type of light sources were confounded in this field study, the results should only be considered as consistent with our theoretical understanding of how regular, 24-h light-dark exposures combined with high circadian light exposures can promote greater behavioral alignment with work schedules and with sleep.
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    • "The light–dark and activity–rest patterns recorded by the Daysimeter can be used to quantify the degree of lightdependent circadian disruption exhibited by individuals using a technique known as phasor analysis. Phasor analysis as applied to measure circadian entrainment and disruption has been described elsewhere (Rea et al., 2008) but, briefly, the relationship between the 24-h light–dark exposure pattern, the stimulus, and the rest– activity pattern, the response, can be quantified in terms of the phase and the magnitude of their joint circular FIGURE 2. Presented in the left panel is the modeled spectral sensitivity of the murine circadian system for narrowband and for polychromatic lights based upon light-induced changes in circadian phase for wheel-running activity. Presented in the right panel is the functional relationship (r 2 ¼ 0.80) between the spectrally weighted levels of circadian light for the mouse and the measured changes in circadian phase following 30-min exposures to different amounts of narrowband light stimuli. "
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    ABSTRACT: Although circadian disruption is an accepted term, little has been done to develop methods to quantify the degree of disruption or entrainment individual organisms actually exhibit in the field. A variety of behavioral, physiological and hormonal responses vary in amplitude over a 24-h period and the degree to which these circadian rhythms are synchronized to the daily light–dark cycle can be quantified with a technique known as phasor analysis. Several studies have been carried out using phasor analysis in an attempt to measure circadian disruption exhibited by animals and by humans. To perform these studies, species-specific light measurement and light delivery technologies had to be developed based upon a fundamental understanding of circadian phototransduction mechanisms in the different species. When both nocturnal rodents and diurnal humans, experienced different species-specific light–dark shift schedules, they showed, based upon phasor analysis of the light–dark and activity–rest patterns, similar levels of light-dependent circadian disruption. Indeed, both rodents and humans show monotonically increasing and quantitatively similar levels of light-dependent circadian disruption with increasing shift-nights per week. Thus, phasor analysis provides a method for quantifying circadian disruption in the field and in the laboratory as well as a bridge between ecological measurements of circadian entrainment in humans and parametric studies of circadian disruption in animal models, including nocturnal rodents.
    Full-text · Article · Sep 2014 · Chronobiology International
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    • "Phasor analysis was used to characterize the relationship between the 24-hour light–dark pattern and the 24-hour rest– activity pattern.34 Since CS is a measure of the effectiveness of optical radiation on the retina in stimulating the human circadian system, the daily patterns of CS were used in the phasor analyses – the larger the phasor magnitude, the greater the correlation between these two rhythms. "
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