For me, it's definitely when I'm not sleeping well. I suffer from pretty gnarly seasonal allergies that have me coughing and sneezing so hard when I am asleep that it wakes me up. I toss and turn and just stare blurry-eyed at the ceiling for hours and hours. During these six weeks of my life I probably only average around four to five hours of sleep a night. Do that for weeks on end and it makes my fluency just take a nose dive. Luckily, my home life and work life are pretty good. That doesn't mean that there isn't stress but at this point in my life I'm not worried about my wife leaving me or getting laid off so I just go with the flow. It's almost entirely exhaustion that does me in every. Single. Time. Luckily, I am on new medication now for the allergies and I have been sleeping well the last week or so. You never miss sleep until you don't get it, then it's the more important thing ever.