Burnout is usually described as having too much to do. I suspect that's only part of the picture.
Another possibility is that the problem isn't simply the amount of thinking, but the point at which thinking stops producing anything useful.
Most difficult problems benefit from sustained attention. Spend thirty minutes thinking through a conflict at work and you'll often come away with a clearer understanding. Spend two hours making an important decision and you may notice considerations that weren't obvious at first.
The interesting part is that the relationship doesn't appear to stay linear.
At some point, the return on additional thinking begins to taper. Another hour may still produce something worthwhile, but probably not as much as the hour before it. Eventually, much of the thinking consists of revisiting the same ideas from slightly different angles. The subjective experience is often that progress is still being made, although objectively the return on that investment may have become quite small.
One question that follows is why intelligent people seem particularly vulnerable to this process.
I suspect it's because thinking has generally worked very well for them. Analytical people are accustomed to solving difficult problems through sustained effort. When additional thought has repeatedly produced better decisions throughout someone's life, it is understandable that the same strategy would be applied when they become stressed, overwhelmed, or uncertain. The assumption isn't irrational. In many situations it is exactly right.
The difficulty arises when the strategy continues after the point at which meaningful progress has largely stopped.
The investment continues, but the return has diminished substantially.
Consider a fairly ordinary example. Imagine receiving a difficult email on Monday morning. Spending twenty minutes thinking through a response is probably time well spent. The reply is likely to be more measured, more accurate, and more useful than whatever might have been written immediately.
Now imagine continuing to think about that same email on Tuesday and Wednesday. The wording is reconsidered. Possible interpretations are reviewed. Future conversations are rehearsed. Explanations are generated for why the sender chose one phrase instead of another.
By that point it becomes less clear that the original problem is still being solved. Much of the mental effort is being devoted to a problem that has already received most of the benefit careful thinking was likely to provide. The investment continues, but the return has diminished substantially.
Clinically, this is one of the ways I tend to think about rumination. It isn't that thinking is inherently problematic. In fact, thoughtful analysis is often one of a person's greatest strengths. The question is whether the thinking is continuing to generate new understanding or whether it has gradually become self-perpetuating.
That distinction seems to matter.
Many professions reward exactly this style of thinking. Physicians, attorneys, engineers, psychologists, executives, and many others are expected to anticipate problems before they occur. Those habits are adaptive in the environments in which they developed. Difficulties tend to arise when the same cognitive style becomes the default response to virtually every uncertainty, including those that no longer benefit from further analysis.
Seen this way, burnout may not simply reflect an excessive workload. In some people it may also reflect an excessive investment in cognitive effort after the useful return on that effort has already begun to decline.
If that's true, the solution probably isn't to think less. It's to become better at recognizing the point at which additional thinking is no longer accomplishing what it once did.