Some people find that doing yoga in the morning energizes their body for the rest of the day, while yoga in the evening can keep them wide awake for hours making it difficult to fall asleep. Others find yoga to be a calming ritual before bedtime that helps them get to sleep quickly.
You may want to practice relaxation techniques prior to going to bed.Try lying down in Savasana (corpse pose) with your eyes closed and focus on slowing your breath. Begin by pulling the air slowly all the way down to your abdomen, and then with an even slower exhalation completely empty the body of air. It may help to count slowly in your mind as you inhale for 3-5 seconds. Then exhale for 4-6 seconds.
Additional resources: AM & PM Yoga Meditations, PM Yoga Meditations