Sleeping!

I have a 5 week old DS and a 14 yr old DD. I am feeling my age this time around I'm not old, just older than when I gave birth to DD.
I have had a few social invites from friends (these are not dinner / cinema / soft social invites they're for full on nights out) and, so far, have been able to side swerve going out by being somewhat honest in so much as I'm just not ready to leave DS yet. And I have also been diagnosed with PND (whole other story) but the real, honest to Godfrey reason is....
I'd rather catch up on missed sleep if free time were available.
I love my friends but I'm just so bloody exhausted and socialising is the last thing I feel like doing.
AIBU? Should I force myself to socially flutterby?