I'm guessing from the question that you're not a commercial pilot so, do you mean on an RNAV STAR or for a normal descent profile?
The aircraft will not just descend, unless you have told it to. ATC have no idea where your profile ToD is and generally (in Europe anyway) you're restricted by STAR levels and speeds so you have to deviate from the "ideal" profile.
If normal profile then once cleared to descend you can push Alt for a managed descent or pull for open descent.
On an RNAV profile it's a little less straightforward depending on where you are and what you're cleared to do.