What are some phrases to express that it is hard to think because you are very tired. In English, saying your 'brain is fried' can be due to external reasons such as taking drugs, but I'm talking very simply about how to say you're tired and that it has become hard to think clearly and/or at a normal pace.
I'd say "У меня мозги кипят" (i.e. "my brain boils"). This could also mean an external reason, as an extreme heat, but usually it stands for "I'm stuck", "I'm tired of thinking" etc.
BTW. This is probably due to driver's slang: if the engine overheats and starts smoking then one says "Мотор закипел".