Is the landside of Austin-Bergstrom ever closed at nights/early mornings?
After security checkpoints close for the night, passengers are restricted to landside only. Passengers who have arrived on a late flight connecting to a morning flight are able to stay in the secure area of the terminal.
A few reviews mention the loud and constant overhead announcements that can be heard through most of the airport (excluding the Mezzanine), so keep some earplugs or noise cancelling headphones handy if you want to sleep!
For uninterrupted sleep, there is a hotel on airport grounds and several other nearby hotels that offer an airport shuttle. See Airport Hotels below.