When considering the best time of day in which to incorporate yoga into your schedule, keep in mind that it is generally recommended to wait one to two hours after eating a meal before practicing yoga asanas (postures) or pranayama (breathing practices). While this can make arranging your schedule a bit more challenging, there are several good reasons for this advice.
First, it takes energy to digest our food. When we exercise right after eating (and this is true of all kinds of exercise, not just yoga), our exercising muscles take energy away from our digestive tract, which interferes with the efficiency of the normal digestive process. In addition, the fact that our digestive system is competing with our muscles for blood flow and energy means that we will not be able to give our practice our best because we will feel more sluggish than usual.
Second, yoga postures involve moving our bodies in ways that can constrict various parts of our digestive tract during the movements. If our digestive tracts are relatively empty, this is not a problem. When it is full of broken down food, these temporary constrictions can cause pressure to build up at those places in the system as the digestive tract continues to try to push the food matter through as it normally would. This can cause discomfort and aggravate existing conditions, like diverticulitis.
On a practical level, eating shortly before your yoga practice can cause nausea, gas pains, bloating, discomfort, or the need to use the restroom during practice. All of these take away from your attention to your practice.
Of course, there will be times when you may find that it is necessary to eat something prior to your yoga practice due to scheduling issues or a feeling of low blood sugar. In these cases, try eating a small amount easily digestible foods, like applesauce, at least 30 minutes before your practice to get you through your time on the mat. Save larger meals for after your practice is complete.