I would say, on arriving and being asked how I am, “I’m really knackered, big week at work but didn’t want to cancel, so am just coming along for a couple of hours, really wanted to see you!” then that paves the way for later. You might find you want to stay longer, though.
This ^
Even when tired though, I would get a 'second wind' from being out, and enjoying my time with friends.
As to whether you are being unreasonable or not depends on the other circumstances. As pps have said, it depends on the circumstances of your friends.
So, if you weren't there, would they be meeting up anyway / will they be quite happy just as the two of them? If so, then YANBU
Have they had to make potentially complicated arrangements to be there (babysitters / swapping shifts / travel etc) ? If so, then YABVU
If this a one off or do you often flake out of arrangements? As that would affect my thinking too.
If you struggle to keep your eyes open, then why not arrange to meet up for lunch rather than go out for the night and then bail half way through?