It's possible that you're not doing a good enough cool down after your workouts. If your heart rate is still increased because of the workout before you go to bed, that would cause you sleeping problems. You should try to find a way to decrease your heart rate i.e. wind down and relax after your workout.
What type of yoga are you doing ? In theory, the yoga class should help you relax and be able to sleep. Of course, if you're doing Bikram or Power Yoga, those could keep your heart rate elevated after the workout.
Try reading and having a cup of tea when you get home to help you relax after the workout. You should try to make sure your heart rate is at a normal level so that you can sleep.
If you find that doesn't help, then you may have to change your workout times to earlier in the day.