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Dark period is a way for sleepers to stabilize their circadian rhythms in the absence of long blocks of sleep and increase the quality of [[Wikipedia:Slow-wave sleep|SWS]]. It also prevents certain health issues from arising. For most schedules, it should start 1-3 hours (2h is recommended in most cases) before the dusk (SWS peak) sleep block and continue for around 8 to 12 hours afterwards.

Dark period is a way for sleepers to stabilize their circadian rhythms in the absence of long blocks of sleep and increase the quality of [[Wikipedia:Slow-wave sleep|SWS]]. It also prevents certain health issues from arising. For most schedules, it should start 1-3 hours (2h is recommended in most cases) before the dusk (SWS peak) sleep block and continue for around 8 to 12 hours afterwards.

== Rationale ==

== Rationale == <!--T:2-->

Before the advent of electrical lighting in modern times, humans historically had a stable period of time during the night in which there was little light. This kept their circadian rhythms stable and synchronized to the natural day/night cycle.

Before the advent of electrical lighting in modern times, humans historically had a stable period of time during the night in which there was little light. This kept their circadian rhythms stable and synchronized to the natural day/night cycle.

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>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.

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>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.

== Timing ==

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

The optimal timing of dark period depends on your schedule. For most schedules, it should begin around 1-3 hours (the default recommendation is 2 hours) before your '''dusk/evening''' sleep (core for [[Special:MyLanguage/Everyman|Everyman]], first core for [[Special:MyLanguage/Dual core|Dual core]], [[Special:MyLanguage/TC1|TC1]], and [[Special:MyLanguage/TC2|TC2]], and evening core for [[Special:MyLanguage/Triphasic|Triphasic]].

The optimal timing of dark period depends on your schedule. For most schedules, it should begin around 1-3 hours (the default recommendation is 2 hours) before your '''dusk/evening''' sleep (core for [[Special:MyLanguage/Everyman|Everyman]], first core for [[Special:MyLanguage/Dual core|Dual core]], [[Special:MyLanguage/TC1|TC1]], and [[Special:MyLanguage/TC2|TC2]], and evening core for [[Special:MyLanguage/Triphasic|Triphasic]].

* For [[Special:MyLanguage/E1|E1]] and [[Special:MyLanguage/Siesta|Siesta]], it should start before core and end immediately upon waking.

* For [[Special:MyLanguage/E1|E1]] and [[Special:MyLanguage/Siesta|Siesta]], it should start before core and end immediately upon waking.

* For [[Special:MyLanguage/E2|E2]], [[Special:MyLanguage/E3|E3]] (including extended), and [[Special:MyLanguage/E4|E4]] it should last until the end of first nap as long as that nap is within 5 hours of the core.

* For [[Special:MyLanguage/E2|E2]], [[Special:MyLanguage/E3|E3]] (including extended), and [[Special:MyLanguage/E4|E4]] it should last until the end of first nap as long as that nap is within 5 hours of the core.

== Mechanism ==

== Mechanism == <!--T:9-->

[[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>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.   

== Methods ==

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

=== Goggles ===

=== Goggles === <!--T:12-->

The most popular and versatile method for achieving a dark period is through the use of red-coloured laser goggles (colloquially known as "DP goggles" in the [[Special:MyLanguage/Polyphasic Sleep Discord|Discord community]]). There are several types of these available, and below is a comparison between their light-filtering lens tints:

The most popular and versatile method for achieving a dark period is through the use of red-coloured laser goggles (colloquially known as "DP goggles" in the [[Special:MyLanguage/Polyphasic Sleep Discord|Discord community]]). There are several types of these available, and below is a comparison between their light-filtering lens tints:

{| class="wikitable"

{| class="wikitable"

=== Red interior lighting ===

=== Red interior lighting === <!--T:14-->

If you live in a space where you are in control of the lighting, it is possible to use either RGB or pure red LED lights during dark period to avoid the inconvenience and/or discomfort from wearing goggles. However, you must still be sure that you wear dark period glasses when looking at screens such as your electronic devices, and if you live in an area with bright lights outside your house, it is best to draw your blinds (blackout curtains ideally) in order to block any external light.

If you live in a space where you are in control of the lighting, it is possible to use either RGB or pure red LED lights during dark period to avoid the inconvenience and/or discomfort from wearing goggles. However, you must still be sure that you wear dark period glasses when looking at screens such as your electronic devices, and if you live in an area with bright lights outside your house, it is best to draw your blinds (blackout curtains ideally) in order to block any external light.

=== Computer screen colour filters ===

=== Computer screen colour filters === <!--T:16-->

{| class="wikitable"

{| class="wikitable"

|+Comparison of colour filter software

|+Comparison of colour filter software

For all of these software packages, you should change the colour temperature to the minimum possible (1000K for redshift, 1200K for f.lux).

For all of these software packages, you should change the colour temperature to the minimum possible (1000K for redshift, 1200K for f.lux).

For Linux on Wayland, please use the night light options offered by your desktop environment (KDE and GNOME offer this feature). Editing the colour temperature manually might be necessary to achieve the amount needed. [https://www.omgubuntu.co.uk/2017/07/adjust-color-temperature-gnome-night-light Change colour temperature on GNOME]

For Linux on Wayland, please use the night light options offered by your desktop environment (KDE and GNOME offer this feature). Editing the colour temperature manually might be necessary to achieve the amount needed. [https://www.omgubuntu.co.uk/2017/07/adjust-color-temperature-gnome-night-light Change colour temperature on GNOME]

'''Note''' that, even with all the red colour filtered away, all screens, with the exception of OLED screens on some phones, '''will bleed backlight''', which is white and can compromise your dark period. Depending on the quality and brightness setting of your screen, this may or may not be an issue.

'''Note''' that, even with all the red colour filtered away, all screens, with the exception of OLED screens on some phones, '''will bleed backlight''', which is white and can compromise your dark period. Depending on the quality and brightness setting of your screen, this may or may not be an issue.

== Living with dark period ==

== Living with dark period == <!--T:20-->

Using any of the above methods to achieve a dark period should limit the accuracy of your perception of non-red lights during your dark period. This may impair you ability to do certain tasks. However, there are some methods to mitigate this.

Using any of the above methods to achieve a dark period should limit the accuracy of your perception of non-red lights during your dark period. This may impair you ability to do certain tasks. However, there are some methods to mitigate this.

=== Monochromatic filters ===

=== Monochromatic filters === <!--T:22-->

Using red goggles or red filters can make it difficult to see certain colors on your screen. To solve this problem, you may wish to temporarily remove colors on your screen with a monochrome/greyscale filter. Here are some ways to do it:

Using red goggles or red filters can make it difficult to see certain colors on your screen. To solve this problem, you may wish to temporarily remove colors on your screen with a monochrome/greyscale filter. Here are some ways to do it:

On iOS and macOS, go to Accessibility > Display > Color Filters, turn on Color Filters and select “Greyscale”

On iOS and macOS, go to Accessibility > Display > Color Filters, turn on Color Filters and select “Greyscale”

On Android, open up Settings, then tap Digital Wellbeing & parental control, then choose Wind Down, you can set up a schedule for the evening, which will turn your screen greyscale for the scheduled time. Alternatively, go to Settings > Developer Options > Simulate colour space and choose Monochromacy. (For this, you need to have Developer Options enabled. [https://developer.android.com/studio/debug/dev-options link])

On Android, open up Settings, then tap Digital Wellbeing & parental control, then choose Wind Down, you can set up a schedule for the evening, which will turn your screen greyscale for the scheduled time. Alternatively, go to Settings > Developer Options > Simulate colour space and choose Monochromacy. (For this, you need to have Developer Options enabled. [https://developer.android.com/studio/debug/dev-options link])

On GNOME, you can use the extension [https://github.com/YurchenkoSt/desaturate_all Desaturate All] (remember to checkout to the feature branch) to turn your display monochrome with the click of a button. It is possible to use switch back and force between red-only and red-greyscale mode to make a rough estimate of the underlying colour (this also applies to other methods).

On GNOME, you can use the extension [https://github.com/YurchenkoSt/desaturate_all Desaturate All] (remember to checkout to the feature branch) to turn your display monochrome with the click of a button. It is possible to use switch back and force between red-only and red-greyscale mode to make a rough estimate of the underlying colour (this also applies to other methods).

On Windows 10, you can use the key combination Ctrl + Windows + C to toggle a grayscale filter. This can be configured by finding Turn color filters on or off from the start menu.

On Windows 10, you can use the key combination Ctrl + Windows + C to toggle a grayscale filter. This can be configured by finding Turn color filters on or off from the start menu.

=== Colour pickers ===

=== Colour pickers === <!--T:28-->

When there is a need to determine some colours with precision, it is best to use a colour picker program. These programs grabs a pixel from your screen and records the brightness value in each of the subpixels.

When there is a need to determine some colours with precision, it is best to use a colour picker program. These programs grabs a pixel from your screen and records the brightness value in each of the subpixels.

=== Prescription glasses ===

=== Prescription glasses === <!--T:30-->

If you need prescription glasses for vision correction, you may wear both at the same time via any of the three methods listed below.

If you need prescription glasses for vision correction, you may wear both at the same time via any of the three methods listed below.

==== Wear them outside the goggles ====

==== Wear them outside the goggles ==== <!--T:32-->

Wearing your prescription glasses outside the goggles is the preferred option for the first and second goggle types on the table, as there is less light leakage.

Wearing your prescription glasses outside the goggles is the preferred option for the first and second goggle types on the table, as there is less light leakage.

==== Wear them inside the goggles ====

==== Wear them inside the goggles ==== <!--T:34-->

It may be possible to wear most styles of corrective lenses inside of dark period goggles, depending on the model of your goggles. The orange and yellow goggles as pictured in the above table can fit over many glasses models, but it may not be particularly comfortable, depending on the shape of the prescription frames.   

It may be possible to wear most styles of corrective lenses inside of dark period goggles, depending on the model of your goggles. The orange and yellow goggles as pictured in the above table can fit over many glasses models, but it may not be particularly comfortable, depending on the shape of the prescription frames.   

==== Glue lenses onto the goggles ====

==== Glue lenses onto the goggles ==== <!--T:36-->

This might be the most comfortable option once completed. Detaching your prescription lenses from their frames and glue them to the top of your goggles. Hot glue will not melt either the lenses or the first two goggles (the others have not been confirmed).

This might be the most comfortable option once completed. Detaching your prescription lenses from their frames and glue them to the top of your goggles. Hot glue will not melt either the lenses or the first two goggles (the others have not been confirmed).

[[File:Lenses on goggles.jpg|thumb|Example]]

[[File:Lenses on goggles.jpg|thumb|Example]]

Tips:

Tips:

* Choose the glasses that match in curvature with your goggles, so it would be easier to attach.

* Choose the glasses that match in curvature with your goggles, so it would be easier to attach.

* Perform the operation in a dry and preferable cold environment to minimize the amount of moisture in the gap. Failure to do so can result in the goggles fogging up in cold environments.

* Perform the operation in a dry and preferable cold environment to minimize the amount of moisture in the gap. Failure to do so can result in the goggles fogging up in cold environments.

Warnings:

Warnings:

* Might cause slight dizziness at first due to axial misalignment. Try holding your prescription lenses in front of and behind your dark period goggles while they're on your face to determine which feels better before gluing them.  

* Might cause slight dizziness at first due to axial misalignment. Try holding your prescription lenses in front of and behind your dark period goggles while they're on your face to determine which feels better before gluing them.  

== Related advice ==

== Related advice == <!--T:40-->

=== Fasting period ===

=== Fasting period === <!--T:41-->

It is recommended that you maintain a fasting period that covers the entirety of your dark period, as food is also an important [[Wikipedia:zeitgeber|zeitgeber]], or circadian cue. Intermittent fasting has been anecdotally suggested to work well with polyphasic sleep. It is preferable to eat during the day rather than at night. A breakfast helps set the circadian day start. Water may be consumed, but any xenobiotic consumables will disrupt your circadian rhythm.

It is recommended that you maintain a fasting period that covers the entirety of your dark period, as food is also an important [[Wikipedia:zeitgeber|zeitgeber]], or circadian cue. Intermittent fasting has been anecdotally suggested to work well with polyphasic sleep. It is preferable to eat during the day rather than at night. A breakfast helps set the circadian day start. Water may be consumed, but any xenobiotic consumables will disrupt your circadian rhythm.

=== Exercise ===

=== Exercise === <!--T:43-->

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>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.

== See also ==

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

* [[Special:MyLanguage/Crimsonflwr|Crimsonflwr]]'s free and comprehensive dark period [https://www.polyphasic.net/courses/dark-period-course/ course] (registration to polyphasic.net required).

* [[Special:MyLanguage/Crimsonflwr|Crimsonflwr]]'s free and comprehensive dark period [https://www.polyphasic.net/courses/dark-period-course/ course] (registration to polyphasic.net required).

* [[Special:MyLanguage/Crimsonflwr|Crimsonflwr]]'s dark period [https://docs.google.com/spreadsheets/d/10dAiiTWo4V6Fq2P0b4_xjSyvdydOHcz68BLxWGyFlSw/edit?usp=sharing calculator]. Refer to the above course for instructions regarding usage.

* [[Special:MyLanguage/Crimsonflwr|Crimsonflwr]]'s dark period [https://docs.google.com/spreadsheets/d/10dAiiTWo4V6Fq2P0b4_xjSyvdydOHcz68BLxWGyFlSw/edit?usp=sharing calculator]. Refer to the above course for instructions regarding usage.

== References ==

== References == <!--T:47-->

<references />

<references />

[[Category:Lifestyle]]

[[Category:Lifestyle]]

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