This article suggests that feeling tired at 3pm is perfectly normal, and should be countered by simply having a nap.
However, in some workplaces, this isn't possible (a lack of beds for one).
I can work flexible hours, so I can choose when to start my day, or when to have my break. One possibility is to start work at 6-7 am, and finish at 3pm.
What is the best way to counter 3pm slump, if you have to be working then?