Translation:My grandmother sleeps on the sofa when she is sleepy.
exactly, think of cansado as meaning exhausted. You can be exhausted (cansado) without being sleepy (tener sueño), or the other way round. They are two different "concepts", although the English "tired" is actually close to cansado, when you think of the verb from which it comes: to tire (from old English tēorian ‘fail, come to an end’, also ‘become physically exhausted’).
I said "My grandmother sleeps on the sofa when she feels sleepy" and *feels" was marked wrong by DL. One translation for Tengo sueño is I feel sleepy - so I feel a bit peeved. I think a report anyway.
Edit: after the report I changed it to "is sleepy" and of course it is accepted. - "feels" should also be accepted for tiene in my view in this case
noarndt, I have the same question that you are asking. Tienes cansado = You are tired. Tienes sueño = You are sleepy. To translate from english: My grandmother sleeps on the sofá when she is tired. Can I use either (tener cansado, or tener sueño)? Very interested to understand the difference if there is one!
The important difference to understand in your example is that cansado is an adjective and sueño is a noun. Tener is ALWAYS followed by a noun ( or a conjunction like "que" ). Remember that "rule" (it's not a rule) when you have questions. I.E. Tengo sueño, tengo calor, tengo sed, tengo dos hijos. Versus: I.E. estoy sueño, estoy calor, estoy sed, etc.
Please forgive me if I am beating a dead horse, but I have one more question. In your examples above you include: (1) Tengo sueño, and (2) estoy sueño. Is the (1) sueño a noun and the (2) sueño an adjective? And, (whoops 2 questions) can the same be said of Tengo cansada and estoy cansada?
Sorry to be confusing. The examples I used with "tener" (1) are the correct way and the "estoy" examples (2) are the incorrect way. Tengo casado doesn't make any sense, but estoy cansado (I'm tired) does. Tengo sueño (I'm sleepy) makes sense , but estoy sueño does not.
Notice that for it to make sense tener is followed by a noun and estar is followed by an adjective.
Does that help?