Ah yes, ,the post-lunch-slump!
I also work 10 hour shifts, and I get this when I have lunch at my desk and don’t physically remove myself from my work area. Whenever I go to the lunch room and get a chance to chat with my co-workers, I find the return to work far less difficult. I also agree, that having a nutritious lunch and an exciting treat (coffee, tea, kombucha) to look forward to, will always help.
Remember that your body and mind aren’t designed to be super productive for 10-12 hours per day. So if you need to ride out that post-lunch haze for 30 minutes, then just be kind to yourself and let it happen.