It's true that many patients with low back pain report their pain levels vary throughout the day. For some patients time of day makes a difference. Others say their pain is worse if they stay in one position too long. Many patients say certain actions increase their pain. This could be bending, lifting, or turning.
In cases like yours, it may be best to report the most common pain level as you think over a 24-hour period. Some forms allow space for comments. You may want to leave it blank and give offer the information directly to the doctor. When in doubt, just ask the office staff or the doctor.