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