Of course it applies to groups of people. The usual definition of nation is largely political. I was referring to that definition.
Poland was a huge state in the 17th century and then gradually shrank to nothing in the 18th only to come back after WW I. Germany and Italy were a collection of different states, whose borders shifted continuously. We think of Ferdinand and Isabella as co-rulers of Spain, but Ferdinand's domains were largely outside of Spain. The Netherlands and Belgium were once one nation, but when they rebelled against the Spanish crown, the Netherlands was successful and became a Protestant nation while Belgium (which the Spanish reconquered) stayed largely Catholic. Shifting political boundraies were the norm until WW II.As to the fluidity of Europe's political boundaries that is a relatively recent phenomenon.
This isn't a very useful definition because how do you determine whose cultures are similar enough to be a common culture? A couple of hundred years ago France contained a number of regions that spoke different languages, with different economic life and cultures. Was the France of the 18th century not a nation? Was the US a nation before the civil war considering the significant differences along these lines between North and South? It comes down to a matter of judgement and that makes the results less reproducible by others and not very generalizable.I would agree that there is a tendency to use political history, however, even if we simply just use economic history we still revert to national cycles. And that has to do with both the nature of nations. Namely: A nation is a historically constituted, stable community of people, formed on the basis of a common language, territory, economic life, and psychological make-up manifested in a common culture.
There isn't a great solution. What I have done is look at the patterns of events in different regions and when they match up, figure these regions must be more or less on the same saeculum timing. An example is the timing of the religious events in the early 16th century showing that most of Europe seemed "ready" for an Awakenng at that time. You can also see this turning in the frequency of peasant revolts and popular uprisings.
Another time when there was a definite burst of unrest was over the 1355-1385 period. After the plague real wages in England were rising because the plague had reduced the labor supply. Since the plague happened all over Europe the same thing was probably happening everywhere. Rising real wages with no increase in labor productivity means hard times for property/business owners. Hence they attempted to use the power of the state to fix wages at pre-plague levels and to restrict labor mobility. It's not surprising that people were po'd and it shows up in the record.
During the Reformation the reverse was happening. Population was rising and real wages falling. People were unhappy about that. This too was manifest by elevated unrest which shows up in the record. It is also associated with an Awakening. It is possible that the Plague forced everyone on the same schedule and the Reformation re-inforced it. There was also a continent-wide famine in 1315-17 that was associated with some evidence of elevated unrest and relgious activity. Spikes in the frequency of religious events in the early parts of the previous two centuries can also be identified tentatively also. So there is some evidence of a saeculum-like cycle extending through multiple nations in Europe from Medieval times. But it is fragmentary and does not rise to the level of statistical signficance.