I wouldn't want to encourage a scruple over it, but in general, you are offering time and work for pay, and you need to be working.
This said, no one can continuously work without either taking some kind of break, or slowing the pace of work as needed. Any employer needs to understand this.
The recent trend is to hire as few people as possible, pay them as little as you can get by with paying them, and get as much work out of them as possible. The mantra is "do more with less" and multitask, multitask, multitask. That's a recipe for getting slipshod work out of people.