That is a great question. I’ve been there too. The two things that work best for me are:

1. Taking power naps. I now take two per day. I wake up at 4am and take my first nap at around 8:30am. The second nap is usually between 2:30pm and 3:00pm. I don’t really schedule those though. I do them when I feel I’m not as productive as I should be. If I wake up later, I usually should take a nap in the afternoon.
2. Meditate for 10–20 minutes using the Headspace app. This helps me clear my mind and focus better after.

As for fatigue, in my case, I don’t really get it because I diversify the areas of the brain that I work. So even though I work about 15–16 hours per day, the biggest mental stretch is during my actual work, between 9am to 7pm. And I nap in between. The other hours are dedicated to physical activities or mental activities that work different areas of my brain, like drawing or learning languages.

I also leave my Sundays free for doing whatever I want. Plus I tend to take “long” breaks after two-three months of intense work. Like 2–4 weeks. I realize though that most people can’t do that.

Hope that helps!