It's hard to know your limit at the outset. Any new project, side job, or work task seems manageable from a distance. Even as you begin a new endeavor, it may go smoothly for a while. But there may come a time when you realize you've blown past your glass ceiling.
We can all push ourselves for brief periods. Like a sprinter, we can deny ourselves sleep, down time, and relaxation with the promise of monetary gain. Then if we see ourselves profiting, we ask our minds and bodies to keep going just a little longer. Just a few more weeks and I'll stop.
Or maybe it's a fear of disappointing someone. The workload at your job is beyond your capacity, but fear tells you not to say anything. What if you get fired? Yes, that's a risk. But continuing at an unsustainable pace is even more dangerous. To your sanity and to your health. I have found that when you're a good worker, honest, and trustworthy, there is nothing to fear. More often than not, you will be met with understanding rather than a pink slip.