Work that should be manageable sometimes feels exhausting for no obvious reason, nothing about the task itself explains it. Research on engagement points to a specific mismatch behind that feeling, not a general complaint about workload, and the two possible causes need different fixes.
The narrow channel where work feels right
Research on flow, a state of full engagement in a task, first developed by the psychologist Mihaly Csikszentmihalyi, finds it occurs in a narrow channel where challenge and skill are closely matched. Too little challenge relative to your actual skill produces boredom. Too much produces anxiety. Both get described the same way in the moment, this is draining, even though the underlying cause is close to opposite.
Workplace research on flow finds it occurs more often, and is rated as higher quality, during work than during leisure, despite people generally saying they prefer leisure. It is also associated with higher wellbeing, more creativity, better performance, and people staying longer at their jobs.
Two causes that feel the same from the inside
Boredom is often the harder of the two to admit, especially where any dissatisfaction with assigned work can read as ingratitude. It helps to name it privately first, before deciding whether or how to raise it with anyone. Anxiety is usually easier to name but harder to fix quickly, since it often means the task currently exceeds what you have capacity for right now, not what you are permanently capable of.
Why the fix has to match the actual cause
If the real problem is boredom and the response is more support or a lighter load, the draining feeling gets worse, not better, since the mismatch moves further from where it needs to be. If the real problem is anxiety and the response is more autonomy or a harder slice of the work, the same thing happens in the other direction. Diagnosing correctly matters more than reacting quickly.
Where to start
Pick the piece of work that has been feeling draining lately, and ask honestly which side of the mismatch it actually is. Is it dragging because it is too easy for what you can actually do, or is it draining because it currently exceeds what you have room for right now.
Once you know which one it is, the useful ask is specific, more autonomy or a harder slice of the work for boredom, more support or a smaller scope for anxiety, rather than a general complaint about being overloaded that does not distinguish the two.
Telling the difference between the two honestly, rather than defaulting to whichever one feels more acceptable to admit, is something you are better placed to judge than any general framework. What this gives you is the two real explanations, so you are choosing between them rather than guessing.
There is a specific way to raise this once you know which one it is, so it reads as a solvable request rather than a vague complaint, and it is worth working out before you bring it to anyone.