The best time of day to practice yoga is before sunrise. Those are the rules of all the yoga books. However this may not work for you and your life. If it means you are never going to practice yoga then you need to find another time of day that works for you. You need to make yoga work for you and your life. If you are not a morning person then practice yoga in the evening. If you have time at lunch then practice yoga during your lunch hour. Yoga has to fit into your lifestyle. If early morning works for you, great! Otherwise, rules are made to be broken, this might be one of them for you.