And this is linked to a number of factors. Eating for instance causes an influx of sugar that puts us in an anabolic, restful state. If you’re having a big lunch then, do it at the start of your break so that you have time to rest and digest literally.
Likewise, recognize that you can’t spend your whole day in focused, alert mode without it taking a toll. Make sure you schedule mini breaks and listen to what your body is telling you.