if you can't think straight, you are not thinking calmly and clearly about something.
I was so tired I wasn't thinking straight any more.There are so many people talking, I just can't think straight.
can't think straight
Cannot think rationally or calmly due to being overwhelmed or overcome with something (often an emotion or exhaustion). Don't assign him anything else right now—he's so stressed about this case that he can't think straight.After working a 36-hour shift, I can't think straight and only want to climb into bed.