The “siesta” is a widespread custom in Spain and increasingly copied abroad
Today every person carries a watch or digital gadget that tells us the time to the nearest second and even more. Punctuality required today in our usual lives is not of hours, but usually of minutes or seconds, which is matched to the exact step of the sun by a given meridian.
In the Ancient World in general and in particular in the Roman world times were less accurate. Say things happened or “ante meridiem”, a.m. (before noon) or "de meridie" or post meridiem, p.m. (after noon), "mane" (in the morning), "vespere" (in the evening), without much more precision.
Romans should not be very interested in accuracy because when the first sundial was brought from Sicily in the year 263 BC, it marked the time wrongly, because it was built for a different latitude. It took them 99 years to correct the mistake, when in 164 BC they built a sundial matched to the latitude of Rome.
The clepsydra or Geek water clock (horologium ex aqua) was not introduced until the end of the Republic.
In these moments the day or light time was divided from sunrise to sunset in twelve hours and the night or dark time in four vigils. Depending on the season, the duration of hours or vigils was rather long and not very accurate.
Hours were called or referred to the ordinal Latin adjective. So they talked about prima hora, secunda hora, tertia hora … (first hour, second hour, third hour…).
Precisely the sixth hour (sexta hora) marked the noon time, when some lucky ones took a nap or slept for a while. Among these fortunate ones was the emperor Augustus, as Suetonius tells us in his “Lives of the Twelve Caesars”, II, 78:
“After his midday meal he used to rest for a while just as he was, without taking off his clothes or his shoes, with his feet covered and his hand to his eyes”
“Post cibum meridianum, ita ut vestitus calciatusque erat, retectis pedibus
paulisper conquiescebat opposita ad oculos manu”
From this expression "sixth hour" derives the Spanish word "siesta", that the “Dictionary of the Royal Spanish Academy" defines as "a dream that is taken after eating or lunchtime", widespread custom in Spain and increasingly copied abroad, to the point that the word "siesta" has been assumed by the British and American people that accurately used it instead of their own word "the nap".