Questions about this exercise “Mindfulness of Sleeping”

Hola Luverrain

Yes, we tend to use the definite article instead of the possessive when it's really obvious, so I imagine you're referring to one of these sentences from the writing challenge: 

voy a tumbarme en mi/la cama

tengo que centrar la atención/mi atención

Using the possessives is not wrong, it would just put more emphasis. As we can't add too much text/hints on the writing challenges we accept both options. 

As per "va a ser una experiencia relajante", we are focusing here in the description of that experience and for that we normally use "ser" the same way we say "ella es inteligente, la silla  es cómoda". 

Saludos