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Modern humans often use electric lighting during the night. This can cause difficulty as the body thinks it is still daytime even if it is already midnight, and is part of the reason why people often experience the desire to sleep later and later each day.<ref>Eun Yeon Joo, Sabra M Abbott, Kathryn J Reid, Donghong Wu, Joseph Kang, John Wilson, Phyllis C Zee (April 2017). ''[https://www.sciencedirect.com/science/article/abs/pii/S1389945716302295 Timing of light exposure and activity in adults with delayed sleep-wake phase disorder]''. Sleep Medicine, Volume 32, April 2017, Pages 259-265, https://doi.org/10.1016/j.sleep.2016.09.009</ref> For monophasic sleepers, a somewhat short dark period can be maintained during their long sleep hours, which keeps their circadian rhythms relatively stable.

Modern humans often use electric lighting during the night. This can cause difficulty as the body thinks it is still daytime even if it is already midnight, and is part of the reason why people often experience the desire to sleep later and later each day.<ref name="Joo2017" /> For monophasic sleepers, a somewhat short dark period can be maintained during their long sleep hours, which keeps their circadian rhythms relatively stable.

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However, for polyphasic sleepers who might not have such long sleep blocks, this poses an issue. As artificial light is present around the clock, [[wikipedia:Melatonin|melatonin]] production is suppressed and the quality of SWS is compromised. This causes extra difficulty in adaptation and is also unhealthy, as circadian disruption is linked to various health issues.<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4632990/</ref> To avoid this, polyphasic sleepers (and monophasic sleepers too) should practice a dark period, during which no blue, green or white (as this contains both blue and green) light should enter the eye.

However, for polyphasic sleepers who might not have such long sleep blocks, this poses an issue. As artificial light is present around the clock, [[wikipedia:Melatonin|melatonin]] production is suppressed and the quality of SWS is compromised. This causes extra difficulty in adaptation and is also unhealthy, as circadian disruption is linked to various health issues.<ref name="pmid25635592" /> To avoid this, polyphasic sleepers (and monophasic sleepers too) should practice a dark period, during which no blue, green or white (as this contains both blue and green) light should enter the eye.

== Timing == <!--T:6-->

== Timing == <!--T:6-->

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[[File:Melanopic.png|thumb]]

[[File:Melanopic.png|thumb]]

The intrinsically photosensitive retinal ganglion cells (ipRGCs) controls the secretion of melatonin by the pineal gland. Their sensitivity to light is independent from normal vision, which is mediated by cones and rods in the eyes. The ipRGCs have a peak sensitivity at 480 nm and drops off on both sides.<ref>CIE TN 003:2015, Report on the First International Workshop on Circadian and Neurophysiological Photometry, 2013, published 2015</ref> During the dark period it is preferable to avoid light altogether, as all light provides some melatonin suppression, but this is not reasonable for most people's lifestyles. Instead, is best to simply avoid as much bright light as possible, and limit light intake to only necessary yellow, orange and red light in order to limit melatonin suppression as much as possible.   

The intrinsically photosensitive retinal ganglion cells (ipRGCs) controls the secretion of melatonin by the pineal gland. Their sensitivity to light is independent from normal vision, which is mediated by cones and rods in the eyes. The ipRGCs have a peak sensitivity at 480 nm and drops off on both sides.<ref name="cie15" /> During the dark period it is preferable to avoid light altogether, as all light provides some melatonin suppression, but this is not reasonable for most people's lifestyles. Instead, is best to simply avoid as much bright light as possible, and limit light intake to only necessary yellow, orange and red light in order to limit melatonin suppression as much as possible.   

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== Methods == <!--T:11-->

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Intense exercise such as strenuous weight lifting and long-distance running should be avoided during dark period, as this might disrupt the circadian rhythm.<ref>Christopher A Wolff, Karyn A Esser, Exercise timing and circadian rhythms, Current Opinion in Physiology, Volume 10, 2019, Pages 64-69, ISSN 2468-8673, <nowiki>https://doi.org/10.1016/j.cophys.2019.04.020</nowiki>.</ref> In addition, exercise may also disrupt sleep and/or increase sleep onset as a result of increased heart rate. However, during adaptation, it might be desirable to use exercise as a means to stay awake. Choose mild exercise routines and do not exercise for an extended period of time to avoid circadian disruption or sleep onset issues; both of which may hinder your sleep quality and harm your adaptation.

Intense exercise such as strenuous weight lifting and long-distance running should be avoided during dark period, as this might disrupt the circadian rhythm.<ref name="Wolff2019" /> In addition, exercise may also disrupt sleep and/or increase sleep onset as a result of increased heart rate. However, during adaptation, it might be desirable to use exercise as a means to stay awake. Choose mild exercise routines and do not exercise for an extended period of time to avoid circadian disruption or sleep onset issues; both of which may hinder your sleep quality and harm your adaptation.

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== See also == <!--T:45-->

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<references />

<ref name="Wolff2019">{{cite journal |vauthors=Wolff CA, Esser KA |date=2019 |title=Exercise timing and circadian rhythms |journal=Current Opinion in Physiology |volume=10 |pages=64-69 |doi=10.1016/j.cophys.2019.04.020 |issn=2468-8673}}</ref>

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