What time of day should I schedule my thesis defense?
First, I have never heard of any research backing up the idea that defenses will be more less successful based on the time of day the defense is held.

Second, the best time to defend is whenever your committee can actually make it. When I defended, my goal was to get get at least three of our four committee members in same physical room (one non-chair member could attend via video). Within a six-week window, I felt lucky to find any two hour-window to schedule the defense.

Finally, you are overthinking things. Pour your energy and concern into the content and remember that if your committee is encouraging you to defend, it's because they think you are ready. Surprises are unlikely.

Of course if you are in the enviable position of having to choose a time of day, I personally like defenses at the end of the work day so that you can head out with others — e.g., students, family members, maybe even committee members — for a celebratory post-defense drink or meal.