If a person in a circle becomes unconscious from exhaustion (I doubt that one can simply fall asleep with the ecstasy of that much of the OP in you), this person can still contribute the OP strength to the link till a possible death of physical exhaustion. If the leader of the circle becomes unconscious, though, the circle likely dissolves, or at least you don't have anyone who weaves something anymore. Cadsuane implies at the Cleansing that only Nynaeve's death would release her from the circle, when Nynaeve almost falls unconscious.