My orthodontist told me to only wear my retainer at night, should I be worried about teeth shifting?

First, congratulations on having your braces removed!  It is such an exciting time, but also a very important phase of your orthodontic treatment.  If you were to survey several orthodontists, each and every one of them may have a different philosophy regarding retainer wear. Since your orthodontist bonded permanent retainers to your upper and lower teeth, night time wear of your upper Essix should be fine, but of course it is best to check with your doctor if you are unsure.