Here is the Purple line schedule.
http://www.transitchicago.com/assets/1/purpleline_schedules/Foster.pdf
The Noyes stop is 2.5 blocks west of NU’s campus. Weekdays, there are express trains that will take one directly to and from the loop. On weekends, the Purple line stops at Howard, and one has to transfer to the red or yellow lines. However, these schedules as supposed to match up so there is little waiting. The trains run until 2:00 AM.