What is the difference between....

I am learning bisaja and am wondering what the difference is between the two sentences.
"Matulog na ko"
and
"Motulog na ko"
I know that basically they mean "I'm going to slee now" but how exactly are MAtulog and MOtulog different?
Thank you!! 

Salamat kaayu!

I think I understand now.  I have a few cebuano friends on facebook and I see on their status updates sometimes "Matug sah ko" - so even though it means that they are going to sleep, and they are using shortcuts (using "tug" instead of "tulog"), is that phrase grammitcally incorrect?