We all have days where motivation is hard to come by. Show the hiring authority that you can keep yourself active, productive, and efficient despite a lack of supervision. The interviewer wants to see that you can regulate yourself without being micromanaged.
With this question, it will be easy to be overly-optimistic; however, an approach of infinite optimism will not seem genuine or realistic. Be honest, upfront, and practical. Even the most disciplined person is not motivated 100% of the time. What matters the most is that you get yourself back in the saddle after giving yourself a motivational check-in.