I’m a fan of the Pomodoro technique: 25 minutes focused work 5 minute break. Two cycles then a longer break (10-30 minutes)
I like to have deep work session early in the morning. It helps me to feel more productive which feeds me with more energy to keep doing high caliber work. Plus, I found that when I do the hardest or most focused work first, it frees up the rest of my day.
However, if I cannot scheduled productive work early, I attempt to do it after a great workout in the evening. A hard and engaging workout typically gives me more energy and helps me to recalibrate mentally.