Dark period
Dark period is a way for sleepers to stabilise their circadian rhythms in the absence of long blocks of sleep. For most schedules, it should start 1.5-3 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 helps them stabilise their circadian rhythms and keeps 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 relatively stable, albeit short, dark period can be maintained during their sleep hours, which prevents their body clocks from going totally haywire.
However, for polyphasic sleepers who might not have such long sleep blocks, this poses an issue. As light is present around the clock, melatonin production is suppressed and the quality of SWS is compromised. This causes extra difficulty in adaptation and is also unhealthy. 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.
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. 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).
