Was your window open? That pretty much voids any sound proofing. If both your and your neighbor's windows were open in a quite night, no wonder you can hear everything.

The neighbors may not have been home and were at work. Which was why you never heard anything until late that night.

Ah well, if it was annoying then, they'd have complained by now. Also, it might just be that you had a window open or something.

Don't be surprised when you find a hat with a dollar or two in it in front of your door. Also... Kinki

Maybe you should get a louder vibrator, so there's no chance you'll hear what they're doing while they listen to you?

It's an apartment. What makes you think someone is going to spend tons of money on sound-proof walls in a building they own to make money? YDI.

I don't think an open window would let in sounds from next door. Unless they had back to back windows from the side,

Silence is golden. But duct tape will always be silver. :P

Maybe your neighbour just yawns loudly. >.> Or maybe you put on a horrible show and he was kindly letting you know.

I suspect foul play here. Something tells me Mr. Neighbor Man was under your bed the whole time.