I used to occasionally worked 80+ hours at a time in my younger days, during a period where I was working for an oil services company, building tooling for migrating different business systems to SAP.

The company has several locations across the globe, and each was using something different to be migrated to SAP. One time I flew to Brazil to build SAP integration tooling for the local division of the company - 3 flights over 20 hours (all in economy), and I was expected to go straight to the office on arrival. I then worked feverishly, drinking gallons of local coffee to sustain me. Everyone else was doing the same. We ended up making some bad decisions (I think because we were all so tired), and at one point I totally lost my shit with the Brazilian project lead, coming close to getting violent. I'm absolutely not a violent person, but that amount of sleep deprivation, pressure and caffeine are not a good combination.

I'd do this for 2-3 weeks at a time, and feel completely fried at the end of each stint.

I simply couldn't do that anymore, but I'm also convinced it was a false economy - if we'd just worked normal hours without all the pressure from above, we could have achieved the same result, without all the mess along the way.