My worst experience was after a huge night out in Cancun and only an hour's sleep. I then took a flight without sleeping to Miami and then onto London. I landed at 6am and turned up at the office to show willing. Unfortunately the .... had just hit the fan and I ended up doing Excel modelling until 11pm.
The last few hours I had to change the resolution down on my monitor as my vision had gotten so bad that I wasn't able to read the numbers.