Effects Of Light In Humans

As in other species, human melatonin production is limited to nighttime darkness. Ambient light exposure suppresses nighttime melatonin production; however, the light needs to be brighter than the intensities that are effective in laboratory animals (Figure 1) (11). To date, the most significant finding related to melatonin has been its role in the discovery that light is of importance in the regulation of human biological rhythms

Figure 1 (left). Effect of light on melatonin secretion. Each point represents the mean concentration of melatonin (± standard error) for six subjects. A paired t- test, comparing exposure to 500 lux with exposure to 2500 lux, was performed for each data point. A two-way analysis of variance with repeated measures and the Newman-Keds statistic for the comparison of means showed significant differences between 2:30 a.m. and 4 a.m. (*, P < .05; **, P < .01). (right)) Effect of different light intensities on melatonin secretion. The averaged values for two subjects are shown. Symbols: Asleep in the dark (open circles); 500 lux (open squares); 1500 lux (closed circles); and 2500 lux (crosses). From Lewy et al. (11) with permission.

Figure 1 (left). Effect of light on melatonin secretion. Each point represents the mean concentration of melatonin (± standard error) for six subjects. A paired t- test, comparing exposure to 500 lux with exposure to 2500 lux, was performed for each data point. A two-way analysis of variance with repeated measures and the Newman-Keds statistic for the comparison of means showed significant differences between 2:30 a.m. and 4 a.m. (*, P < .05; **, P < .01). (right)) Effect of different light intensities on melatonin secretion. The averaged values for two subjects are shown. Symbols: Asleep in the dark (open circles); 500 lux (open squares); 1500 lux (closed circles); and 2500 lux (crosses). From Lewy et al. (11) with permission.

(A. Lerner, personal communication). Light suppression of melatonin is also a useful test for light sensitivity (12-14).

Light exposure resets the ECP according to a phase response curve (PRC) (15). PRCs to light are typically characterized by a "dead zone" of reduced responses in the middle of the day. In the first half of the night, light exposure causes phase delays and in the second half of the night it causes phase advances. The magnitude of these phase shifts increases towards the middle of the night, at which time there is a crossover time that separates delay responses of large magnitude from advance responses of large magnitude. During the middle of the day, there is also a crossover time that separates advance responses of small magnitude from delay responses of small magnitude.

The first suggestion that there was a PRC to bright light in humans came from studies providing or removing bright light in the morning just after awakening or in the evening just before bedtime (16-17); notably, the sleep/wake cycle was held constant. The first evidence for a crossover time in the middle of the night came from a field study of jet lag (18). Subsequently, four complete PRCs have been described (19-22) that are in general agreement with each other and with the one originally hypothesized (23-24).

Along with the early assessments of phase-shifting responses to morning vs. evening light in normal controls, patients with circadian phase disorders were also treated with either morning or evening bright light exposure (24-25). There are two types of circadian phase disorders, the phase-advanced type and the phase-delayed type. In the former category are advanced sleep phase syndrome and east-to-west jet lag. In the latter category are delayed sleep phase syndrome and west-to-east jet lag. Most shift workers also fall into these two categories. Those who are abnormally phase advanced should be exposed to evening light, and those who are abnormally phase delayed should be exposed to morning light.

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