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Phase delaying the human circadian clock with a single light pulse and moderate delay of the sleep/dark episode: no influence of iris color.

Canton JL, Smith MR, Choi HS, Eastman CI - J Circadian Rhythms (2009)

Bottom Line: There were no significant correlations between any of the iris color variables and the magnitude of the phase delay.There was no evidence that iris color influenced the magnitude of the phase shift.Future studies are needed to replicate our findings that iris color does not impact the magnitude of light-induced circadian phase shifts, and that the previously reported differences may be due to race.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biological Rhythms Research Laboratory, Department of Behavioral Sciences, Rush University Medical Center, Chicago, IL, USA. ceastman@rush.edu.

ABSTRACT

Background: Light exposure in the late evening and nighttime and a delay of the sleep/dark episode can phase delay the circadian clock. This study assessed the size of the phase delay produced by a single light pulse combined with a moderate delay of the sleep/dark episode for one day. Because iris color or race has been reported to influence light-induced melatonin suppression, and we have recently reported racial differences in free-running circadian period and circadian phase shifting in response to light pulses, we also tested for differences in the magnitude of the phase delay in subjects with blue and brown irises.

Methods: Subjects (blue-eyed n = 7; brown eyed n = 6) maintained a regular sleep schedule for 1 week before coming to the laboratory for a baseline phase assessment, during which saliva was collected every 30 minutes to determine the time of the dim light melatonin onset (DLMO). Immediately following the baseline phase assessment, which ended 2 hours after baseline bedtime, subjects received a 2-hour bright light pulse (~4,000 lux). An 8-hour sleep episode followed the light pulse (i.e. was delayed 4 hours from baseline). A final phase assessment was conducted the subsequent night to determine the phase shift of the DLMO from the baseline to final phase assessment.Phase delays of the DLMO were compared in subjects with blue and brown irises. Iris color was also quantified from photographs using the three dimensions of red-green-blue color axes, as well as a lightness scale. These variables were correlated with phase shift of the DLMO, with the hypothesis that subjects with lighter irises would have larger phase delays.

Results: The average phase delay of the DLMO was -1.3 +/- 0.6 h, with a maximum delay of ~2 hours, and was similar for subjects with blue and brown irises. There were no significant correlations between any of the iris color variables and the magnitude of the phase delay.

Conclusion: A single 2-hour bright light pulse combined with a moderate delay of the sleep/dark episode delayed the circadian clock an average of ~1.5 hours. There was no evidence that iris color influenced the magnitude of the phase shift. Future studies are needed to replicate our findings that iris color does not impact the magnitude of light-induced circadian phase shifts, and that the previously reported differences may be due to race.

No MeSH data available.


Related in: MedlinePlus

Circadian rhythm phase delays with a single bright light pulse and delayed sleep/dark episode. The horizontal lines represent the median phase delays.
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Figure 2: Circadian rhythm phase delays with a single bright light pulse and delayed sleep/dark episode. The horizontal lines represent the median phase delays.

Mentions: The average phase delay of the DLMO was -1.3 ± 0.6 h, and the median phase delay was -1.4 h. There were large individual differences in phase shifts, with one subject delaying as little as 3 minutes, and others delaying about 2 hours (Fig 2). The average baseline DLMO was 22:22 ± 1.3 h, and the average baseline DLMO to baseline bedtime interval was 1.9 ± 1.3 h. The phase angle between the baseline DLMO to start time of the light pulse ranged from 2.3 to 6.5 h, and was similar for subjects with blue and brown irises. The correlation between this phase angle and phase shift of the DLMO was r = .51, p = 0.08, indicating a tendency for subjects receiving the light pulse closer to their DLMO to have larger phase delays.


Phase delaying the human circadian clock with a single light pulse and moderate delay of the sleep/dark episode: no influence of iris color.

Canton JL, Smith MR, Choi HS, Eastman CI - J Circadian Rhythms (2009)

Circadian rhythm phase delays with a single bright light pulse and delayed sleep/dark episode. The horizontal lines represent the median phase delays.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2722576&req=5

Figure 2: Circadian rhythm phase delays with a single bright light pulse and delayed sleep/dark episode. The horizontal lines represent the median phase delays.
Mentions: The average phase delay of the DLMO was -1.3 ± 0.6 h, and the median phase delay was -1.4 h. There were large individual differences in phase shifts, with one subject delaying as little as 3 minutes, and others delaying about 2 hours (Fig 2). The average baseline DLMO was 22:22 ± 1.3 h, and the average baseline DLMO to baseline bedtime interval was 1.9 ± 1.3 h. The phase angle between the baseline DLMO to start time of the light pulse ranged from 2.3 to 6.5 h, and was similar for subjects with blue and brown irises. The correlation between this phase angle and phase shift of the DLMO was r = .51, p = 0.08, indicating a tendency for subjects receiving the light pulse closer to their DLMO to have larger phase delays.

Bottom Line: There were no significant correlations between any of the iris color variables and the magnitude of the phase delay.There was no evidence that iris color influenced the magnitude of the phase shift.Future studies are needed to replicate our findings that iris color does not impact the magnitude of light-induced circadian phase shifts, and that the previously reported differences may be due to race.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biological Rhythms Research Laboratory, Department of Behavioral Sciences, Rush University Medical Center, Chicago, IL, USA. ceastman@rush.edu.

ABSTRACT

Background: Light exposure in the late evening and nighttime and a delay of the sleep/dark episode can phase delay the circadian clock. This study assessed the size of the phase delay produced by a single light pulse combined with a moderate delay of the sleep/dark episode for one day. Because iris color or race has been reported to influence light-induced melatonin suppression, and we have recently reported racial differences in free-running circadian period and circadian phase shifting in response to light pulses, we also tested for differences in the magnitude of the phase delay in subjects with blue and brown irises.

Methods: Subjects (blue-eyed n = 7; brown eyed n = 6) maintained a regular sleep schedule for 1 week before coming to the laboratory for a baseline phase assessment, during which saliva was collected every 30 minutes to determine the time of the dim light melatonin onset (DLMO). Immediately following the baseline phase assessment, which ended 2 hours after baseline bedtime, subjects received a 2-hour bright light pulse (~4,000 lux). An 8-hour sleep episode followed the light pulse (i.e. was delayed 4 hours from baseline). A final phase assessment was conducted the subsequent night to determine the phase shift of the DLMO from the baseline to final phase assessment.Phase delays of the DLMO were compared in subjects with blue and brown irises. Iris color was also quantified from photographs using the three dimensions of red-green-blue color axes, as well as a lightness scale. These variables were correlated with phase shift of the DLMO, with the hypothesis that subjects with lighter irises would have larger phase delays.

Results: The average phase delay of the DLMO was -1.3 +/- 0.6 h, with a maximum delay of ~2 hours, and was similar for subjects with blue and brown irises. There were no significant correlations between any of the iris color variables and the magnitude of the phase delay.

Conclusion: A single 2-hour bright light pulse combined with a moderate delay of the sleep/dark episode delayed the circadian clock an average of ~1.5 hours. There was no evidence that iris color influenced the magnitude of the phase shift. Future studies are needed to replicate our findings that iris color does not impact the magnitude of light-induced circadian phase shifts, and that the previously reported differences may be due to race.

No MeSH data available.


Related in: MedlinePlus