What time of day should I schedule my thesis defense? With apologies for the attention-grabbing title: of course I am not actually asking you to tell me when I, specifically, should schedule my defense. But I have heard very plausible-sounding rumors that the time of day at which a PhD candidate holds their thesis defense can have an impact on the difficulty and even the candidate's chance of passing. For example, scheduling a defense just before lunch may mean the committee will ask fewer and less involved questions, because they will be anxious to finish so they can go eat. Similarly defending in the mid afternoon may lead to less complex questions because everyone is tired - or alternatively it could cause a more difficult defense because the committee members are in no hurry to get back to work. And so on; the rumors abound.

What I would like to know: is there is any research backing up the idea that there are better or worse times of day for a candidate to schedule their thesis defense? Any anecdotal experience from people who have sat on multiple PhD committees?

Of course I'm not claiming this should be a major influence on when one schedules their defense, but people do talk about it, so (as a scientist!) I can't help but wonder if there's any truth behind the idea.
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.