In what sense? Are you restoring over an existing database, or creating a new one. If the latter, there is no downtime, as such. If an existing one, I assume that the existing database won't be updated during the restore period, as it would be lost. If so, you could restore the database under a different name, then, once the restore is complete, rename the old database and then rename the new one (to the old one's name).
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.