WASO (wake after sleep onset) is defined as an unwanted wake or waking period after sleep onset. By definition, sleep must continue after the wake in order for it to count as a WASO. A wake after sleep onset, after which sleep does not continue, is called a premature wake.
WASOs occur more frequently during polyphasic adaptation and are thought to be caused by the brain figuring out the new behavioral state times, as well as a sign for repartitioning. In this framework, they are thought to cause no problems and are viewed as a part of polyphasic adaptation.
In addition, there are people with the natural tendency to wake up at night, staying awake for 2-3 hours, and then falling back asleep. People experiencing this frequently are naturally inclined to dual core schedules. In addition, it has been observed that humans automatically develop a segmented sleep schedule, without the need for alarms or other restrictions, if the duration of the dark period is about 14h long.
Furthermore, WASOs can be caused by bad sleep hygiene, including caffeine consumption, as well as by advanced age and can be a sign for many sleep disorders, such as sleep apnea.
Typically, WASOs reduce sleep quality dramatically, making them a frequently used parameter for sleep quality in scientific research. Nevertheless, WASOs can cause the opposite effect, namely better sleep quality, if the gap between the WASO and the next sleep onset is 2-3h long, and only if the dark period is not interrupted and sleep is not restricted by an alarm.
Occuring only or much more often during polyphasic adaptation, if minding the adaptation guidelines, WASOs are not a big deal and can neglected as part of adaptation. However it is usually hard to deal with chronic WASOs during the night, especially if the WASOs are age-induced. Exercise and good sleep hygiene, as well as yoga could reduce WASOs.
Another way to treat WASOs effectively, and in fact exploiting them, is using polyphasic sleep. Depending on the amount and length of the WASOs, the fragmented sleep blocks can be adjusted to a fitting length and be integrated in a schedule.
For about one wake during noctural sleep, with long lasting alertness afterwards, picking a schedule from the dual-core line is most fitting. For 2-3 frequent WASOs, a schedule from the Tri-core line can be chosen and for even more frequent WASOs a quad-core schedule might be the best option.
Note that a decently long waking gap needs to be scheduled in order for each schedule to be effective.
For people with chronic WASO-insomia, they can even take place after polyphasic adaptation, but ideally the wakes do not last long and can be neglected if no sleep deprivation symptoms arise.