Most road races start early in the morning to avoid heat and traffic complications. If you do most of your hard workouts in the afternoon or evening, it’s worth shifting an occasional one to the morning so that you have some practice at running fast so early. Most of the bodily functions involved in fast running peak in the late afternoon, so you need to teach your body how to function at a high level at what it considers an inconvenient time.
Early-morning workouts once a month will also give you a chance to experiment with matters such as how soon before running hard you should get up and what your stomach can tolerate then.