I spend long days in the office and while I'm super productive in the mornings, 5am to 1pm is my best time, I tend to crash around 3pm. That is my least productive time when I'm least alert and creative. Then it picks up again around 6pm when the office is empty and I can focus. However, I cant just "not work" at 3 o'clock so here are some of my favorite tips to beat the slump, that dont involve caffeine:
1. Drink two BIG glasses of water! Sometimes you're tired because you're dehydrated. Drinking two big glasses of water helps revitalize you, get rid of any headaches, flushes out toxins, and kickstarts your system again.
2. Brush your teeth! This is one of my favorites when I'm feeling less than awesome. The minty flavor wakes me up, I feel clean and fresh, and ready to take on the next meeting, project, etc.
3. Take a walk or walk the stairs. Getting your blood pumping will undoubtedly get your energy back up, and if you do it in combination with the previous two pints, you wont ever need that afternoon coffee again :)
Good luck, hope this helps makes you productive all day everyday!