I have been studying my own work day for years. I must readily admit that no matter what I do, I am least effective early in the morning. I also discovered that I need time to recover from lunch. Consequently, I have created scheduling parameters where morning appointments start at 9:30 and afternoon appointments start at 1:30.
I have also noticed that I suffer a lull around 3:00 or 4:00. For years, I thought I was just being plain lazy during those times, but Japanese sleep researchers have termed this low phase of afternoon productivity as “the breaking point” or the period of the day when most of us feel the highest level of fatigue.
If you notice this, do something about it: rest, nap, get a snack, go for a walk. Do what suits you, but recognize you can’t go full speed all the time any more than Usain Bolt can run a sub 10 for more than 100 meters. See The Power of Full Engagement by Jim Loehr and Tony Schwartz.