The nightly is built at 6:30AM IndianStandardTime (GMT+5:30), and contains all changes synced till then. The most prominent difference between these builds and the official ones is that there builds are compiled using the Linaro toolchain. To me, this results in a smoother UI, and better battery life than the official builds. From time to time, these builds may also contain some hotfixes for small problems on official builds, and they may also contain some cherrypicks.
You can just install these builds over official builds without a data wipe. You can even go back to the official builds without a data wipe.
Download link: https://copy.com/vJqpoVqwoUXJ
Please check MD5 before flashing.