Afternoon Slump

Lately I've been getting really tired around 4-5pm ish. I don't head into work until 3pm these days and I only work until 6pm. Before I went in from 1-4pm and I was fine. I don't know why the sudden change in mood. I sleep 8 hours a night (usually 9:30pm or 10:0pm to 5:30am to 6:30am). It may that I have to wait to go to work now so I tire myself out? I don't know. Any tips out there? I always make sure to drink at least 72 oz of water a day and get enough sleep and I work out in the mornings.