It depends on the kind of work. If it's routine stuff, past seven hours or so, I can keep going and not feel tired, but I increasingly don't want to, and the feeling that I'd rather be doing something else becomes very distracting. If the work is technical and intellectually rewarding, I might feel inspired to continue, but I start making mistakes and past a certain point, it becomes counterproductive. If the work requires conceptual or creative insights, my brain stops delivering them for free and my backup methods for squeezing them out start failing too. If I'm speaking or writing, I lose the thread and my words lose their punch and personality.
My occasional bouts of insomnia bring a different kind of all-encompassing fatigue. I become overemotional. At my most sleep-deprived, I struggle to operate a kettle. Things were different when I was younger. Writing up my PhD, I essentially slept every other night for months, yet stayed sharp, productive.