During library hours, the 23-hour area is open to the public, though intended for students, staff, and faculty.
Outside library hours, access is limited to UAF affiliates; the doors are locked and can be opened only with active PolarExpress cards. Although the room is often called an "all hours" study area, it is not intended as a living space, and users may not spread their belongings out or eat or drink messily. Also, Campus Police walk through the 23-hour area nightly and remove users who are found sleeping.
If you are a UAF student or employee, then yes, you may stay in the 23-hour study room until 4 a.m.; if you aren't, then no, you may not.