Not every subject, or revision exercise needs the same level of concentration. Going over familiar topics is less intensive than working through past paper questions. Similarly, our ability to focus varies throughout the day.
If you can recognise your mental peak, you can plan to tackle the hardest revision exercises when you’re likely to get the most out of them. Trying to force yourself to a high level of focus when you’re lethargic is likely to make you procrastinate and avoid studying altogether.