I just suggested he go to bed earlier, but he started overreacting as if I were criticizing him.

overreacting 是动词 overreact 的现在分词形式。overreact 由前缀 over-（表示"过度"）和动词 react（反应）组成，字面意思是"反应过度"。 overreacting 在这里表示"做出超出正常程度的强烈反应"，通常带有负面含义，指对小事做出过激反应。