I'm not exactly sure what you're asking, but Spain is not the only culture where napping is customary in the afternoon.

The Yanomamo of the Amazon Basin come to mind (see Napoleon Chagnon's work). Many local cultures in Italy encourage naps (Naples, for example; Ficca, Gianluca, et al. "Naps, cognition and performance." Sleep medicine reviews 14.4 (2010): 249-258.) Greece is another place where afternoon napping (especially for males) is a widespread tradition (see Lee, Kathryn A., et al. "Circadian rhythms and sleep patterns in urban Greek couples." Biological research for nursing 9.1 (2007): 42-48, and also Soldatos, C. R., M. G. Madianos, and I. G. Vlachonikolis. "Early afternoon napping: a fading Greek habit." Sleep 82 (1982): 202-205.)

As with Greece, the habit is fading in modern times in Spain (but lingers in many smaller towns; see George Collier's work).

There are many habitual afternoon nappers in China:

Zhang, G., Fei-Fei Sun, and J. Liao. "The effects of lunch time napping on habitual nappers' mental work efficiency in the afternoon and early evening: an empirical study from China." Health 1.4 (2009): 284-289.

Samoans also have a custom of afternoon napping (whereas some other Polynesian cultures do not).

In Mexico, siestas are more common in mestizo communities than in indigeneous ones, not sure what that means but apparently the Spanish custom didn't catch on completely in Mexico.

One scholar links the practice (or non practice) of siesta in Peru to social class, but also to disease:

Barone, T. Lynne. "Is the siesta an adaptation to disease?." Human Nature 11.3 (2000): 233-258.

In this last article, the author finds an association between agricultural labor, hot weather and siestas.

That article is available on scholar.google if you're interested, and uses HRAF to study the cross-cultural practices of napping in the afternoon. It is mostly in farming-based societies that it develops, but it is certainly not just Spain.