Many Spanish words come from Latin, and siesta is no exception. It comes from the Latin words ‘sexta hora’ - the sixth hour - which means noon, if you start counting at dawn. Taking an afternoon nap became much more common in Spain after the Civil War ended in 1939. There was a shortage of well-paid jobs, so men would often go to work at two different jobs, one in the morning and one in the afternoon, going home at midday to eat and rest. Here are some useful phrases:
echarse una siesta to have/take a nap
la hora de la siesta siesta time
Spain may be famous for naps and long lunches, but all that could change. New laws were recently introduced changing working hours in the public sector, and perhaps bringing an end to the famous siesta.