I, and others, recommend a lightweight down or synthetic fill sleeping bag. As you will be sleeping indoors and not in a tent, a bag rated to maybe 50 degrees Fahrenheit (10 c) should be adequate.
Personally, having to use the toilet several times each night, I recommend a rectangular bag with a full zipper. This makes the bag easier to get out of...when ya gotta go...
Alternatively, consider a good sleeping bag liner made of microfiber material. They are machine washable and should be warm enough for most folks. If it is too "cold," and blankets are not available to toss over your liner, consider wearing the clean clothes you will walk in tomorrow.
After several failed efforts to find the "perfect" microfiber liner, here is the one I use:
http://www.alpsmountaineering.com/products/bags/sleeping-bag-liners/rectangle-liner
It is also available as a mummy shaped bag, and as a rectangular baby in microfleece. But the URL shown above is the most roomy, easily accessed and comfortable bag I have found to date.
This bag folds up small enough to be stored in a one-gallon or three-liter ziplock bag.
Hope this helps.