Having lunch on the desk is a must do. The fact that you have lunch outside the office will leave you with the same atmosphere. And talk to other people. It is a real relaxation during the day.
Lunch outside the office, we have a rest. And take away from the momentary job. This is a brain reset during the day. Boost performance in the afternoon. It also helps reduce obesity. Eat lunch while doing other things. We often eat more than necessary.