Mine is only bad like that when it's misaligned-- either it's up past the cervix or doesn't cover the cervix. My cup is large enough that my cervix sits inside, so as long as it's in there, I only have overflows. But when I was using a smaller cup so alignment was more important, I had more problems at night. The "sealing against the vaginal walls" thing never really worked for me, especially at night when I'm moving into weird positions.

This happened to me last night! I have a small Lunette and don't really know what I did differently, but it was sort of epic. Not only did my cup fail to do its job, but my flow also avoided the cloth pad I was wearing for backup and made it through my underwear, pants, sheets, and got on my mattress, something I've never experienced before. I do know that my cup usually sits very high when I take it out, higher if it's been in longer, and this time I could reach it no problem, which has happened only a few times in the 2.5 years that I've been using a cup. I'm tempted to get a larger cup for night so positioning is less important. Maybe that will work for you?

Yep, stained my matterss too once :( ENOYING!
It is difficult to get that stain off!
Plese read my suggestion on this page.

I used to have a cup that was fine during the day when I was upright but shifted and leaked like it wasn't there at all whenever I'd lie down. I replaced it with another that fit better. Would a second different size/shape cup work better for your overnights too? http://sizecharts.livejournal.com/

On my heaviest nights I usually leak, but that's because I sleep for more than three hours at a time and that's how often I have to change my cup on my heavy days. So for me it's more about capacity than anything else.

I recently found a tip for this same problem I had with my Lunettes. One commenter said that you have to gently pull down the cup (better if you pull on the stem) once it is open fully unfurled inside you. Pull only a tiny bit and this somehow increses the suction. This way it is more dificult to break suction when lying down and getting up.
I triued this for myself and there has been a dramatic improvement with my overnight experience using my cups.