Frequency of commits

Adding files and committing changes to Git is cheap. What happens if you made some great work during the day and, at 5 pm when you were tired you made some bad changes. How can you go back to the state that was 5 minutes ago?

• Commit after adding a new function.
• Commit after writing a new test case.
• Commit after making any small change.
• Commits are cheap and fast.