Something to consider: lunch never truly begins and ends at lunch hours.
In the run-up to lunch, peoples' blood sugar is low, making them less active and creative. This affects everyone, from construction workers to programmers to - scarily enough - court justices.
From The Economist: I think it's time we broke for lunch…:
The team found that, at the start of the day, the judges granted around two-thirds of the applications before them. As the hours passed, that number fell sharply (see chart), eventually reaching zero. But clemency returned after each of two daily breaks, during which the judges retired for food. The approval rate shot back up to near its original value, before falling again as the day wore on.
As for lunch itself, if everybody is eating at roughly different time periods, it squeezes the time where collaboration can actually happen to the first and last few hours of the day. This is especially important in work cultures where talking about work at lunch is a no-no. For example, at my current job, the only rule for our department's monthly lunch outings is no talking about work. Whereas when I worked at Apple, discussion of any work topics at lunch was strictly forbidden - especially among members of other teams.
Now that lunch is over, everyone settles into their own little food coma and slows down as their bodies pump up the insulin to take care of all of the glucose suddenly rushing through their system. This results in the post-lunch slump. VICE described this scenario well, if a bit characteristically crudely:
That said, you know you can turn a lunch hour into a three-hour break, right? Here's the thing, nobody does any work in that precious, golden hour after they've had their lunch. Everybody goes out, eats a meal deal, then has a bit of an hour-long food coma, and a little look at popular websites such as VICE dot com. You do this. And you think you're the only one, but you're not—your boss is doing it, your colleagues are doing it, and %#&@ing Sandra is doing it.
So here's how you turn your lunch hour into a three-hour break: You take it at 12 PM. You have a sandwich, trawl some shops, then settle back at your desk at 1 PM, when everyone else goes on lunch. With them out the way, you can doss off to your heart's content for an hour, and then when they come back and do their hour-long doss, you can also doss because you're not being policed. By 3PM the working day is basically over anyway, so you can just coast your way to 6PM. "Why is Britain dying, Joel?" people ask me. "Why is the economy dying?" I do not know.
So, in short, while it is annoying that your boss wants to strictly define the lunch hour timespan, it is a perfectly reasonable option given the drag that it can have on performance. If the blowback to morale is worse than the opportunity cost of productivity, it may be worth bringing up. Otherwise, it might be fair to say that your boss has a point.