Helicopter's flying over my house in the middle of the night?

I live in the bay area, near SF, and at 12:03 I heard one or more helicopters fly directly above my house. It is now 12:40 sharp, and I hear more and more helicopters. It is as if there is a few doing circles around my area or there are many crossing over this area. Does anyone have any idea as to what so many helicopters are doing in the bay area in the middle of the night? Also, they are SUPER quiet. If I was asleep, I would not have awaken.