Is this part grammatical? Hello dear native speakers Here is a part of a passage taken from TOEFL, and I doubt if a part of it is grammatical : According to conventional theory, yawning takes place when people are bored or sleepy and serves the function of increasing alertness by reversing, through deeper breathing, the drop in blood oxygen levels that are caused by the shallow breathing that accompanies lack of sleep or boredom. Unfortunately, the few scientific investigations of yawning have failed to find any connection between how often someone yawns and how much sleep they have had or how tired they are. About the closest any research has come to supporting the tiredness theory is to confirm that adults yawn more often on weekdays than at weekends, and that school children yawn more frequently in their first year at primary school than they do in kindergarten. What does " About the closest any research has come to supportingthe tiredness theory is...." mean? Is that part grammatical? I feel sth is missing. Here is the link : https://wk.baidu.com/view/5574797e4a7302768e9939f3?pcf=2 Thanks in advance