expression, godan verb (-ru)
think quickly; be sharp-witted; get one's head around
Describes someone's mind working quickly and efficiently, often in problem-solving or understanding complex things. Can be used in both positive and negative contexts (e.g., 頭が回らない for mental fog).
彼は頭が回るから、難しい問題もすぐに解決できる。
He's sharp-witted, so he can solve even difficult problems quickly.
寝不
I couldn't think straight from lack of sleep and even got simple calculations wrong.