Dark period
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 SWS. It also prevents certain health issues from arising. For most schedules, it should start 1-3 hours before the dusk (SWS peak) sleep block and continue for around 8 to 12 hours afterwards.
Rationale
Before the advent of electrical lighting in modern times, humans historically had a stable time during the night in which there is little light. This helped stabilize their circadian rhythms and kept them synced to the natural day/night cycle.
Modern humans often use electric lighting during the night. This can cause difficulty sleeping, as the body thinks it is still daytime even if it is already midnight, and is the reason why people often experience the desire to sleep later and later each day. For monophasic sleepers, a somewhat short dark period can be maintained during their 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 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.[1] To avoid this, polyphasic sleepers (and monophasic sleepers too, for extra benefits) should practice a dark period, during which no blue and green light should enter the eye.
Timing
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 Everyman, first core for Dual core, TC1, and TC2, and evening core for Triphasic.
- For E1 and Siesta, it should start before core and end immediately upon waking.
- For E2, E3 (including extended), and E4 it should last until the end of first nap.
- For Segmented and Dual core, it should cover the entire core gap (gap between first and second core for Tri core), so long as it does not exceed 12 hours.
Mechanism
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.[2] Based on this, in order to avoid melatonin suppression, only yellow-orange and red light should be allowed during the dark period.
Methods
Goggles
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 Discord community). There are several types of these available, and below is a comparison between them:
| Filter type | Specification | Transparency | Verdict | Sample Image |
|---|---|---|---|---|
| Dark red | 200-540 nm @ 99.99% min | data not found | Recommended | 
|
| Light red | 200-540 nm @ 90% min | data not found | Recommended | 
|
| Orange | 
|
30% | Recommended | 
|
| Yellow | 
|
50% | Acceptable | 
|
| Light yellow | data not found | 85% | Avoid | 
|
Notes:
- The dark red goggles may be too dark and reduce vision, but this would useful for use during daylight hours or in bright environments.
- The light red goggles let in substantially more red and green light than the orange one, but less yellow. The overall brightness appears to be slightly higher for the orange one, but the orange one allows for more colour perception. Based on the melanopic sensitivity function, both should provide similar levels of protection against melatonin suppression.
- There is a significant price difference between the first two and the third and the fourth one. However, the third and fourth ones have better quality (fewer rough edges on the plastic, more uniform transparency, and provides more options).
Red interior lighting
If you live in your own room, 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.
Computer screen colour filters
| Name | OS support | Automated? | Note |
|---|---|---|---|
| Redshift | Linux(X11) | Yes | configuration |
| F.lux | Windows | Yes | faq link |
| iOS&macOS native feature | iOS, macOS | No | tutorial |
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. 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.
Living with dark period
Using any of the above methods to achieve a dark period would virtually remove your ability to see all non-red lights. This can impair you ability to do work. However, there are some methods to mitigate this.
Monochromatic filters
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 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. link)
On GNOME, you can use the extension 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.
Colour pickers
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
If you need prescription glasses for vision correction, you need to wear both at the same time.
Wear them outside the goggles
This is the preferred option for the first and second goggle types on the table, as it does not leak much light.
Wear them inside the goggles
This may be an option depending on the model of your goggles. The orange and yellow goggles as pictured in the above table can fit over your glasses, but it may not be particularly comfortable, depending on the shape of your prescription goggles.
Glue lenses onto the goggles
This might be the most comfortable option once completed. Obtain your lenses by detaching them from the glasses, 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).
Tips:
- Choose the glasses that match in curvature with your goggles, so it would be easier to attach.
- Seal the gap completely around to prevent moisture from getting in, as it will be extremely difficult to remove afterwards.
- 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:
- Might cause slight dizziness at first due to axial misalignment.
- The glue would not be easy to remove once attempted.
Related advice
Fasting period
It is recommended that you maintain a fasting period that covers the entirety of your dark period, as food is also an important 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.
Exercise
Intense exercise such as lifting and long-distance running should be avoided during dark period, as they might disrupt the circadian rhythm.[3] In addition, exercise may also disrupt sleep, which is usually in close proximity during dark period. 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 for this purpose.
See also
- Crimsonflwr's dark period course (registration required)
- Crimsonflwr's dark period calculator. Refer to the above course for instructions.
References
- ↑ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4632990/
- ↑ CIE TN 003:2015, Report on the First International Workshop on Circadian and Neurophysiological Photometry, 2013, published 2015
- ↑ Christopher A Wolff, Karyn A Esser, Exercise timing and circadian rhythms, Current Opinion in Physiology, Volume 10, 2019, Pages 64-69, ISSN 2468-8673, https://doi.org/10.1016/j.cophys.2019.04.020.
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