What's the longest time you have slept without waking up?

After traveling from London to San Francisco while being up for 24 hours plus the 8 hour jet lag, I went to sleep last night at 10PM and woke up at 4PM the next day, a total of 18 hours of uninterrupted sleep.

I figured if this was the longest i've slept, I figured you guys must be able to one up me pretty easily. So reddit, what's the longest you've slept at one time without waking up and going back to sleep, and why is it you were able to sleep so long?