It doesn't just happen to 40somethings. It happens to 20somethings too. There is a crow that CAAAAAW CAAAAAAW CAAAAAAWS outside my 9week old baby's bedroom window at 4am which is just what I need isn't it? Thankfully she usually goes back off to sleep once I have scared the bleeding bird off (I have three cats, not that you'd think so) but I can often be found pegging out washing, preparing dinner, sweeping or mopping during the early hours.
So, in short, sorry I can't help with advice on how to go back to sleep but I can sympathise :-)