In CircleCI 2.0 with workflows you are able to schedule a nightly build.
If you are using CircleCI 1.0 or CircleCI 2.0 without workflows, you can trigger a nightly build using the our API.
Which One Should I Use?
The benefit of using CircleCI 2.0 with workflows is that we schedule and trigger the build for you. If you use our API then you would have to set up a cron on one of your own servers in order to trigger a nightly build.