Who knew this was a thing?

That tan piece of cloth with the bunny hork on it is part of an ingenious solution to help alleviate Simon's newly recognized separation anxiety.  Yeah, we didn't know it was a thing either until one evening I was preparing some food for the next night's dinner and was out of Simon's line of sight.  Within 10 minutes of me "not being there" he started to thump rather loudly and insistently (much like he does whenever I would go to bed at night).  It was then that I noticed the pattern of timing and this only ever happening at night after the sun had gone down...

I asked my husband to do a quick Google search for rabbit separation anxiety (so I could still be out of his line of sight) and darned if there weren't similar stories from other people.
As an aside, you can see the foam furniture padding we placed under his cage to help damp the noise a bit (along with their kitten brush and the ridiculous amount of fur they produce during a heavy shed in the baggie--hey, it keeps it from going everywhere and gets tossed when it's full).

We found some tips and tricks to help lessen this all night random thumping that has been going on since November.  Here's the link to my original post:  http://bunnies-and-sunshine.blogspot.com/2012/11/no-simon-thats-bad-simon.html
Quite honestly, we were at the end of our rope at this point and I had no problem giving the little fuzzball the shirt off of my back if it would help solve the problem (I had only gotten a few hours of sleep per night for over a week, so the 2 hours of interrupted sleep the night prior due to the thumping had me wanting to cry I was so frustrated with him). 
It was clear that he wasn't afraid of anything or in pain, just that he wanted (or needed)  my attention.  We were both so tired of the thumping that our gallows humor kicked in and resulted in giving Simon the motto:  "When in doubt, thump it out!" 
What you see Simon playing with is a tee shirt that has my scent on it (I literally gave him the shirt off of my back that evening).  He never chews holes in it like the other cloth pieces in his cage, just nibbles it to shape it and gets bunny hork all over it before snuggling with it like a security blanket.

The combination of the shirt and ignoring the thumping (not giving him positive reinforcement for that behavior) has really helped decrease the night time thumping.
I'll be honest, the first night made us wonder if this was doing anything at all (I think it got worse because he realized we were ignoring him), but over the next few days, the frequency and duration of the thumping started to decrease and after about a week, we only noticed him thumping in the morning when someone's alarm went off/a light turned on so he knew one of us was awake (and he tried to demand early pellets from us).  This has gone on for a couple of months now, so I really think this combination of tactics is working for us.
We're still working on stopping him from making the morning noise, but it's happening less often now and we've managed a few nights strung together of no thumping at all, so that's a start.  The only time we find his behavior getting a bit worse is when we have visitors or there's some change to the normal routine, but it reverts back to normal within a few days.
Uninterrupted sleep is awesome!