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1.
to access and use any of the features of Myspace when you are half asleep, drunk, high, exhausted, unconscious, dead, or otherwise inhibited.

Also used loosely to describe the almost uncanny habit of signing online and unconsciously signing into myspace and checking your messages before doing anything even when you know you don't have any.

"dude i woke up this morning and had half a blog posted before i realized i was sleepspacing"

"Kenny got drunk last night and sleepspaced a whole bulletin about Mary's boobies"
by chrisjzitkovich December 11, 2006