Bella Spa is open till 5am, however, if its busy, they sometimes go till the last client is out (7/8am - Usually Fri/Sat are busiest as the crowd starts arriving at 2am).
Not exactly close to Yonge/Dundas, but just thought Id throw it out to any other interested late nite patrons as Im usually out late myself and would appreciate any more late night information. As for safety, the nieghborhood seems fine. I usually park on the sidestreet just behind the spa, but I think you can park on danforth right infront if you choose. Theres a waiting room, and then each room has a lock. So it seemed safe to me.