For me I find that the post-lunch slump is worse if I eat certain things – e.g. a big stodgy meal. Don’t know if it might be experimenting with different types of food?
Also I find that if I spend my lunchbreak slumped and scrolling through social media on my phone, I don’t feel particularly rested or particularly keen to restart my day. Whereas doing things like going for a short walk outside make me feel happier and more energised (I if can manage to motivate myself to get outside – easier said than done sometimes).
Also, are you doing enough to look after yourself throughout the rest of your day? 10-12 hours of emergency work is a pretty gruelling shift. Are you able to take a couple of little rest breaks throughout the day as well as your lunch break? Are you drinking enough and keeping well hydrated? It could just be that you are working so hard that when you stop to take your mid-shift lunch break your body is exhausted?