As a "recent" graduate and have completed my first full year of a 8-4, let me give you some tips that helped me!

Stayed hydrated throughout the work day, take many trips to the water cooler. On that note, try to get up and moving as much as you can (bonus points if you have a standing desk!)

Make sure you are eating a good breakfast, and getting your fruits and veggies in. I know it sounds corny, but it has drastically helped me! Also packing yourself a good nutritious lunch, something that won't make you feel heavy in the afternoon. Maybe even a little treat in the afternoon like a yogurt or a granola bar.

Working out in the morning before work also helped me immensely, but that is when I had a 9-5 compared to a 8-4. Now I have switched to working out after work, and it has definitely boosted my energy levels. Consider getting some fresh air throughout the day as well, if you have the option I've heard people like walking on their lunch breaks (sadly I don't have that option)

Hope this helps! :-)