My voice seems to get more free and I can lighten up by the end of a night. I've really worked at conserving and pacing myself during a show. It may sound fatigued depending on the gig circumstances. Late nights loud rooms talking to fans wears me out more than the actual singing.
The better your vocal monitoring system can rise above the din of the rest of the band, the more opportunity you have to sing more lightly and let your voice soar rather than crash.
By the time you realize that your voice is going into distortion, it is usually too late, and your cords will suffer from abrasion. Just a little too much and they are going to swell. When they swell, you lose your upper midrange, the money notes. Gone, for however long before you get it back. Your mileage may vary.
When you can hear the monitors clearly without stressing your cords to get loud enough to be heard, the quality part of your voice will be much more resilient. You know, the part of your voice that really gets you excited and causes you to go for more and more high notes.... that part... goes first... when you push too hard... when you're excited... and having too much fun.... for just a few seconds too long....
That's why, like Scott said, PACE YOURSELF. Like Ken says. Pace Yourself.
Don't go all stops out, all night long. Mix it up, start out moderately, do a more challenging tune, ease off for a tune or two, then ratchet up a notch or two... Be fully warmed up and firing on all twelve cylinders before you get to your really hot stuff!!!
You are not a Marshall Amplifier. You can't Wail on Eleven All night long without blowing a gasket!!! Sing a ballad or two. THEN wail like a banshee!