It is possible that the adapted state is unstable or gets destabilized for a sleeper. Some events, injuries, or substances may throw off the sleeper from being adapted. There are a few types of sleep destabilization or reasons that one may find themselves in an unstable adapted state, which are described below.
Low total sleep time
It is usually very difficult to adapt to schedules with a low total sleep time, since much more sleep compression should occur, leaving significantly much less light sleep. This makes the sleep schedule much less flexible and generally unstable, because any event or substance which temporarily increases the sleep needed to recover can easily compromise adapted state. This is likely the main reason why long term polyphasic sleepers more often are those who stay on a schedule which reduces little-to-no sleep, while most nap-only adaptations are shorter.
Also, schedules which reduce sleep greatly require much denser blocks of sleep during the day because of the wake time reduction. Shorter wake gaps are required, which makes such schedules inconvenient for daily life and thus unstable long term. These types of schedules may be destabilized by missing naps too many times or flexing too much/too far due to scheduling conflicts that arise due to the frequent sleeps.
Increased sleep needs
Sleep needs can be increased temporarily, usually as an effect of needed physical recovery. Such cases include increased physical activity and some substances such as cannabis or alcohol. External events such as these sometimes may significantly increase sleep needed (mostly SWS) which can shake the adapted state with increasing intensity inversely proportional to the total sleep time of the schedule. This is the main reason why schedules with at least 3 sleep cycles are commonly recommended in the polyphasic community for those with intense physical activity, as they more likely guarantee the possibility to accommodate enough SWS in core(s).
In some cases, REM sleep can be increased as well. Causes may include depression or intense mental activity. The latter is still controversial, because other studies have shown no connection between REM duration and mental activity such as studying intensity.
Polyphasic sleep schedules usually become flexible after adaptation. The potential extent of one's flexing ranges and frequency depends on the scheduled total sleep time and the stability of their adaptation. Schedules with lower total sleep time are generally much less flexible than schedules with higher total sleep time, as flexing is known to decrease sleep quality which is unacceptable for the schedules with extreme compression. However, this varies greatly from person to person based on sleep needs, duration of stable adaptation, and experience with flexing. When sleep is flexed too often and/or too far, the adaptation can be destabilized or even ruined if the compression cannot be maintained anymore.
An adaptation to flexing is required, and one should only flex once they have gradually increased the flexing range over time once the sleeper is adapted to the schedule and all other flexed sleeps, and their sleep is stable. Flexing too much or too fast is likely to destabilize adaptation as well, which happened to some members of the polyphasic discord community.
Sleep time changes
In a similar vein to flexing, a schedule may be destabilized due to sleep time changes. Examples include the time shift that occurs as a result of Daylight Savings Time, time zone changes as a result of travel, or changing sleep times without a proper flexing adaptation first. These shifts can cause a desynchronization between a sleeper's circadian rhythm and their schedule, which cause similar changes to sleep quality that flexing may have, but often on a greater scale. The farther one shifts a sleep, and the more sleeps they shift at once, the higher their chances of schedule destabilization are.
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