Learn Spanish (Latin America): Nos acostamos a las diez.

Nos is a reflexive pronoun that tells us 'we' are doing the action to 'ourselves.' The verb acostarse (to go to bed) literally means 'to lay oneself down.' For daily routines like this, Spanish uses reflexive verbs, so nos acostamos translates to 'we go to bed.'

I thought acostarse had an o to ue stem change. Why is it acostamos instead of acuestamos?

You are right that acostarse is a stem-changing verb (for example, 'I go to bed' is me acuesto). However, in the present tense, the nosotros (we) form never gets a stem change. It keeps the original o from the infinitive, so it stays nos acostamos.

Where is the word for 'we'? Can I say Nosotros nos acostamos?

Yes, you can add nosotros at the beginning. But because the verb ending -amos and the pronoun nos already make it perfectly clear that 'we' are doing the action, native speakers usually drop the word nosotros.

Why do we say a las diez and not just a diez?