This is serious business. I am working on this information. But I will venture the following:
Foz do Iguaçu
On the Brazilian side shops are open from 8h a.m. to 6h p.m.
Office hours normally from 8h a.m. to 12h and from 1h or 2h p.m. to 6h p.m.
Supermarkets and shops forbidden to open on Sundays. Restaurants downtown mostly close by 2h.
Shops and offices will open from 8h a.m. to 12h and from 4h p.m. to 08 p.m. Siesta time is a must. Exception major supermarkets, bus station, and vital services police, border patrol though some officer do dose a little. Dining time late (11h p.m. onward to 2 a.m.)
Ciudad del Este
Ciudad del Este's is a bit more complex.
The 'Microcenter' or the 'city heart' visited mostly by Brazilian shoppers has opening hours of its own and does not follow official Paraguayan opening and closing times. Shops and other businesses tending to the Brazilian public opens as early in te mornig as possible. It is not unusual to see shops open at 5 p.m. Closing time for shops in the 'microcenter' starts around 3 p.m. By 4 p.m. street vendors, smalls stands and saller shops will be either closed or closing. This is not true for bigger shops and shoppings catering to five-star type of tourist clients and shoppers. Some of these shops will open up to 10 p.m. But and this is some but, these are special arrangements. Check before going. Outside this microcenter enclave the Ciudad del Este of normal Paraguayans follow the national opening and closing hours which includes making a stop afer lunch and being back around 3h. Paraguayans Siesta time is becoming shorter than the Argentine one.