Thesis defense months after thesis submission
In my culture the thesis defense is just a formality. Basically when the supervisor allows the thesis submission he is sure that you had produced more than enough to receive the title (publishing papers, attending to conferences, etc). 

Thus, even if you are not at the best shape for the oral proof, probably it will be enough to be approved. However, slipping the answers of one or two questions could be the difference between the top qualification or the second best.

In my university we have three qualifications for approved theses: "Cum laude", "Magna cum laude" and "Summa cum laude". And it is attributed the status of "Approved by unanimity" or "Approved by the majority".