There is no single best time to take fiber, as it depends on your routine. Consider taking fiber five to eight hours before your usual bowel movement, since it takes time to reach the colon. If you usually poop in the morning (as is common us due to a higher level of a hormone called cortisol), fiber is likely best timed later at night. If you are typically going in the afternoon, fiber may work better for you if taken in the morning. If you have urgent diarrhea multiple times a day, it may be best to spread out the fiber dose. Finding the best time may take experimentation, but consider your typical pattern and ensure at least 8 ounces of liquid whenever you take it.