The task that fits the calendar may not fit the energy left beside it.

An hour on paper is not always an hour you can use. Two demanding things scheduled back to back both fit the day perfectly and still fail, because the second one needs a version of you that the first one has spent. Planning by time alone treats every hour as interchangeable, which is plainly untrue by five in the afternoon.

When you schedule something, ask what will come immediately before it. Put the work that needs a clear head where your head is clearest, and give the flat parts of the day to the tasks that tolerate tiredness. Some things simply need a different hour rather than more determination.