No pienso que + subjunctive
 

“I don’t think that I will go to bed early until I’m tired.”

No pienso que me acueste temprano a menos que esté cansado.

Is it essential to use the subjunctive here after no pienso que, even though me acueste is in the first person singular?

Many thanks in advance.

Yes, it is.
You can drop the 'que' and use the infinitive 'acostarme' if you'd rather, since there is no change in subject.

I agree with Rusty. It's better to use the infinitive form, because for many native speakers the use of the subjunctive with the same subject is not just "clumsy", but incorrect.

Thanks guys.
So, just to confirm, if I decide not to use the infinitive, and use "No pienso que me acueste" - is it true that this is grammatically correct, but not used very often?

Would the same apply to:
"Espero que apruebe mi examen." (I hope that I pass my exam.) I think this sounds grammatically incorrect? (Even though I would use "Espero aprobar..."

Thanks.

If the subject is not changing, native speakers will use the infinitive.
In your second example, your listener will not hear "I hope I pass my exam." They will instead understand "I hope he/she passes his/her exam," unless you were NOT recently talking about another individual. If you weren't already talking about a third person, they would wonder why you didn't use the infinitive. It isn't grammatically wrong, but will be ambiguous for your listener.

Thanks so much for your excellent explanations!