From Polyphasic Sleep Wiki

Consistency refers to the strictness in which sleepers follow their scheduled sleep timings. Most people are able to vary sleep times significantly on Monophasic, with little to no ill effect. However, when adapting to a polyphasic schedule, especially a reducing one, sleep timings must be followed precisely. It is currently not clear as to exactly how strict it needs to be, but it has been suggested, both in Puredoxyk's Uberman adaptations and the DST gradual adaptation experiment, that the body may be sensitive to timing changes as little as 5-10 minutes. It is therefore recommended to follow sleep timings as strictly as possible, with minimal variation on a day-to-day basis.

Consistency is necessary because adapting to reducing schedules requires repartitioning, which is only possible when the sleep timings are stable. In contrary, there has been successful adaptations to non-reducing schedules without strict timings.

After adaptation, it is possible to also adapt to flexing, in which one gradually learns to vary sleep times after first adapting to the schedule. The extent to which this is possible depends on the schedule and individual ability.