Dog Tired

Has anyone ever wondered where the term 'dog tired' came from?  I have just had a shower and crawled into bed with all intentions of going to sleep because I have to get up at the God awful time of 5.30 am tomorrow.  This is of course, all in aid of work, which means that although I am currently weary, I will be 'dog tired' by tomorrow afternoon.  I have a friend's birthday tomorrow evening as well and have sent her an sms because she still hasn't sent me an email with all the details.  She's not one to spend a lot of time on the computer, so it really is forgivable that she may have forgotten she'd promised to email me.  At this time of year everyone has forgetful brain syndrome!

So, anyway, if anyone has a logical reason for the term 'dog tired' or even 'bone' tired', please let me know.  These random nothings do often take up the limited brain space I have.  Well, glancing at the clock it's now 9.14 pm and whilst it's lovely and quiet here (no one's at home), and the rain is playing it's symphony on the colorbond roof, I'm off to visit the land of nod.