Things that I find overwhelming can trigger a very physical feeling of tiredness for me. It doesn't feel like a psychological issue; it feels like I need a friggin' nap. But over the years I've learned that various psychological pressures can make me feel this way. So first, look at whether you are dealing with issues in your writing. Writer's block? Feeling overly self-critical? Overwhelmed with the size of a project, or a looming deadline, or any other problem that stresses you out? This is one possible cause.
I don't have a cure, I'm afraid. Once the feeling sets in, it can be very difficult to overcome without the nap. The best medicine is prevention, breaking it down into small steps and setting small, well-defined goals before you sit down to write.
Another possibility is that you are not writing at a good time of day for you. I'm useless in the evenings. I will stare at my screen and get nothing done. I have to work in the morning or it isn't happening. I would love to be able to use the last few hours of the day, and maybe someday I'll figure it out. (Coffee's out - ruins a night's sleep without any improvement in concentration. I hate that jittery tired feeling.) But anyway, in the meantime, I do everything I can to get myself at the computer when my mind is still functional. This summer it means waking up really early, which sucked at first, but you get used to it.
Diet and exercise are important, of course. Make sure you aren't sacrificing self-care to make time for writing. It always backfires eventually.
Depression can also cause a lot of fatigue. If you are depressed, you are more likely to be overwhelmed, which would bring me back to my first point. So do a little mental health check.
If tiredness is affecting all of your daily functions, and not just when writing, then that is a bigger issue.