I think it depends on how your are....
Some people will be fine come afternoon, like for me I'm like living death in the morning, but really perk up so I'm quite thrilled to do afternoon exams, where as my friend is the complete opposite.
If you are quite alert in the afternoons I think you would be fine, to cram a few concepts and then just read some summaries and do gentle revision for the rest of the day, or if you find yourself quite lethargic, make sure you get plenty of sleep, and then just do gentle revision, you could even revise in the morning and take a nap early afternoon to make sure you're alert.
I don't think say doing an exam will help, and could stress you out leading to losing beneficial marks that you normally would not lose.