Gaps

Gaps are the times between sleeps (cores and naps alike), and for schedules with naps, they are ideally to be minimized for optimal homeostatic pressure management. The longer the gaps are, the more pressure can build up, meaning the surrounding sleeps have to be longer. This is a large part of why monophasic sleep needs more TST than M1, despite M1 not needing to contain vitals in the nap.

A gap that is too long for the surrounding sleeps to buffer it makes a schedule unadaptable.

Minimum length

Gaps must NEVER be smaller than one hour so as to avoid sleeps becoming "interrupted cores", which would mean they would be treated as one by the body and repartitioning / phase order rules break down.

Even above the 1h mark, this can still happen, and is relatively sporadic, and so a minimum gap of 2h is recommended, although 1.5h can work for some.