Since Sam was comfortable and at ease, Dean was hesitant to move away until he absolutely had to. He stayed there until he could feel dawn approaching and his body started to get heavy and sluggish. Much longer and he'd end up fried. That would not be pleasant for Sam to wake up to.

He reached for Sam's arm and started to gently disentangle him so he could slip away. Tried to be as slow and gentle as he could, hoping not to wake his brother.