Keeps a clear head - Idioms by The Free Dictionary
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keep a clear head
To maintain a rational manner, often because one is not under the influence of alcohol or drugs. You need to keep a clear head when you make a big decision like that.
have/keep a clear ˈhead be able to think clearly, especially because you have not had any alcohol, drugs, etc: Don’t give me any wine, I must keep a clear head for the meeting this afternoon. OPPOSITE: a thick head