I find myself asking a lot of inward-directed questions lately, about complacency and challenges and career fulfillment. There's a difference between being "challenged", and being overworked.
If you think you're overworked, you probably are, and despite what others may think, this statement is fundamentally true. Being overworked is, as much as anything, a state of mind. If you really, really enjoy what you're doing, you're ecstatic that you actually get paid for it too. Spending 80 hours doing something you love is nothing...by most standards, people working 80 hours a week would be 'overworked'.
By contrast, if you have to spend even 41 hours a week doing something that is grueling, and don't see any relief or reward in sight, you're going to start feeling overworked. You're going to develop a sense of resentment toward your situation, your employer, your boss, your co-workers who don't carry the same weight you do, etc.
Vacations only serve to postpone & temporarily numb these sensations. They don't go away until something changes for the better. Either the light at the end of the tunnel gets brighter, the incentive to continue drudging on changes or improves, the duties or workload is altered to present new challenges, or you jump ship alltogether in search of something new.
The question remains, if you go through all of that and are still miserable, is the grass really greener anywhere else? The only consistent factor in one's misery - in this situation - is usually themselves. It is this place that I desperately hope I am not.
The sad thing is that, especially with talented people, companies that cannot keep them consistently challenged are not getting the return on investment that they could. When talented people get bored, you can argue that it's the fault of management that they aren't sufficiently stimulated. You can also argue that the individual's attitude and understanding of corporate life should allow them to absorb temporary (even year-long) lulls in exciting work. In fairness, reality is somewhere in between. But you can only keep a dog on the porch for so long...even good dogs.