I'm in the same boat where 6pm comes and I actually have a hard time shifting gears from writing code or thinking intensely about my project. I've been working on my focus and starting earlier in the day. The difference is when I get on track earlier I still chug along until the end of the day. Then by night I'm exhausted and go to bed early.

Work/life balance is important so I should probably listen to my body and focus when it wants to.