It's when you feel like you've got too much work to go to bed, but you are too tired to get anything done. So you just surf the net and procrastinate until you can't stay awake anymore.
Joe: Dude how did your 3000 word essay go?
Tom: Well I managed to get half of it done yesterday, but then I got stuck in sleepytime limbo and just stayed awake the whole night browsing through facebook...
Joe: that sucks bro
When you feel like you have too much work to go to bed, but you're too tired to get anything done so you just surf the web and procrastinate until you can't stay awake anymore.
Joe: Oh man, I'm so tired.. I need coffee.
Dave: Up late again? Are you finally getting your work done?
Joe: No man, I was just stuck in sleepytime limbo again, as usual.
Dave: Oh, damn, I feel you.