A nap is indeed
una siesta. It and its diminutive
siestecita are the only words I've ever used for a nap. That said, '
echar un sueño' is used and means the same thing as the proposals given above, but is no more precise. (Taking a nap always means that you shut down. In Hispanic countries, taking a nap has been culturally extended to include shutting down business, but that probably had its humble beginning in the fact that most were sole proprietors of their businesses and couldn't take a nap and run their business at the same time.
)
'
Sueñito' and '
pestañita' are also good diminutives to use if you want to steer clear from '
siesta', but its only definition in all the dictionaries I have is 'nap'.