Segmented
Mechanism
Segmented sleep, sometimes referred to as Bifurcated sleep, is one of the earliest polyphasic schedules. It is also one of the 4 Biphasic schedules available. Its creation (non-reducing) was natural and dated back to the preindustrial era (1750-1850), making it roughly 250 years old as of today. In short photoperiods (less daytime hours and more night time hours), it was researched that human sleep is also naturally segmented. What is commonly observed in this Segmented behavior is that people would often go to bed after sunset, wake up after some hours at night, stay awake for a couple hours, and then go back to bed until sunrise hours. Segmented sleep is still common nowadays, being involuntarily or voluntarily practiced by many in the world. Nowadays, naturally Segmented sleepers have a short wake gap between 2 cores (~1.5-3h) as a result of interrupted monophasic sleep, in the middle of the night. It only makes sense then, that sleepers would wake up feeling refreshed from the first core, do something at night, before they go back to sleep when sleepiness level rises enough.
Currently, the Polyphasic Discord Community has discovered more sleep mechanics regarding Segmented sleep. For starters, Segmented is one of the 5 schedules with only core sleeps (2) and no naps. The entire basis of Segmented sleep is constructed based on the sleep peaks. It is widely regarded that the early hours of the night (~9-10 PM, SWS peak), when natural melatonin secretion is kickstarted, quality SWS will largely benefit if sleep time begins around these hours. Throughout a lot of EEG readings of adapted sleepers, it is commonly seen that the first core sleep is highly SWS-oriented, while the second core sleep, being near sunrise hours (~6-9 AM, REM peak), has a strong propensity for densely populated REM sleep. The segmentation of the main monophasic core sleep into 2 core sleeps that align with the sleep peaks bolster the sleep quality with much deeper core sleeps, and allow for some potential sleep reduction with strict bedtime everyday. Since 2 core sleeps are already allocated to night hours (which is similar to monophasic sleep's distribution of sleep), no naps in the day are needed, because the amount of light sleep from both cores is sufficient to sustain wakefulness for the entire day.
Even though it may look odd that both core sleeps last for 3.5h rather than the would-be 3h, or 4.5h, it was later explained that each core consists of 2 full sleep cycles plus an extra 30 minutes of sleep time to cover the statistically likely REM period (just like 5h and 6.5h core). The statistically likely REM period originates from Polyphasic Society, and the evidence for it is very poor (eight monophasic readings). While its existence is questionable, it is still possible to see the effects from having the cores be 3.5 hours long. Another explanation for the 30m extension on each core is from the possibility that the cycles extend to roughly 105m as compared to the standard 90m cycles in monophasic sleep. This would explain the viability of the schedule despite having fewer cycles.
The wake gap between each core should be at least 90m, and even more (~2.5-3h) for non-natural Segmented sleepers to ease the adaptation. The first thing is to avoid the interrupted sleep issue. Interrupted monophasic sleep often occurs when there is a only a brief period of awakening during sleep, before the sleeper goes back to sleep. Staying awake for at least ~90m will separate the interruption and create an entirely new sleep architecture for the second core. Should the wake gap be too short, it will be unproductive to do different activities and it may be difficult to fall asleep in the second core as sleep pressure is not high enough to induce sleep. Some people have quit Segmented with a 2h gap because of the inability to adapt or feeling too unproductive during these short night gap.
Difficulty
Before attempting this schedule, it is important to look over all Biphasic patterns to choose if Segmented sleep is worth the choice. For naturally Segmented sleepers, the adaptation is basically complete if they already have a long exposure to the core-split mechanics. For non-natural Segmented sleepers, adapting to Segmented is potentially more challenging than to a biphasic pattern with a long, uninterrupted core sleep at night, such as Siesta and E1. It is still possible to adapt to Segmented, but initially waking up from the first core can pose common issues (e.g, heavy sleep inertia, boredom during the night gap). The second core sleep, being replete with REM sleep, can cause certain amounts of sleep inertia, grogginess or even slight dizziness upon waking up, due to the effects of the dense eye-movement activities from REM sleep. Over time, with enough consistency, these issues will no longer persist, and waking up will be invigorating from each sleep. However, compared to naturally Segmented sleepers, there can still be certain glimpses of mild unproductivity during the night gap after adaptation, and it may require an extended amount of time on the schedule to fully enjoy the night gap.
Thus, the difficulty level of Segmented varies across individuals, and depends on the amount of sleep being scheduled. Overall, it is still considered a friendly schedule to begin sleeping polyphasically, as total sleep is among the highest of all schedules. While potentially not as popular as Siesta and E1, it does promise several niches that completely set it apart from its Biphasic counterparts. This will be discussed in the Lifestyle Considerations section.
