Where there are curtains they do not serve the same purpose as in the UK, at least not around Alicante and Murcia regions. They are not to block out light, deter snoopers or insulate the house. Drawing them is not the way to signal that your day is over and that you have locked yourself, snug and warm, into your little house. In Spain I think they are there to make the casements look a bit neater from inside and maybe to ensure that the dash from bathroom to bedroom is just a shade less embarassing for any neighbours who happen to be watering their plants as you drip by your window.
Spanish houses and flats do have built in blinds or shutters though. Holidaymakers who drop the blinds in their hotel room often find themselves getting up at 11 the next morning when they intended a crack of dawn excursion. The sun may be blazing away outside but the room is a hermetically dark box where it is perpetual Bo-Bo's time.
One of the odd things though is that people don't, necessarily, raise or lower them with daylight. We've often presumed a house is closed up and empty because the blinds never move. Then, one day a burst of activity alerts us to life behind the shutters. In fact, when I think of it, we only raise and lower two of the seven shutters we have here.