How far into my pregnancy should I be sleeping on my left side?

http://www.americanpregnancy.org/pregnancyhealth/sleepingpositions.html

Sleeping on your back: This can cause problems with backaches, breathing, digestive system, hemorrhoids, low blood pressure and decrease in circulation to your heart and your baby. This is a result of your abdomen resting on your intestines and major blood vessels (the aorta and vena cava).

Sleeping on your stomach: When you are farther along in your pregnancy, your abdomen undergoes physical changes and makes it more difficult for you to lay on your stomach.

http://www.babycenter.com/404_whats-the-best-sleep-position-during-pregnancy_7608.bc

sleeping on your left side may benefit your baby by improving blood flow – and therefore nutrients – to the placenta. It also helps your kidneys efficiently eliminate waste products and fluids from your body, which in turn reduces swelling in your ankles, feet, and hands.

Of course, staying in one position all night isn't likely to be comfortable, so turning from side to side while favoring your left side is probably the best strategy.

As for sleeping on your back, avoid that position throughout pregnancy, especially in the later months. Here's why:

When you're sleeping on your back, the weight of your uterus lies on the spine, back muscles, intestines, and major blood vessels. This can lead to muscle aches and pains, hemorrhoids, and impaired circulation, which is uncomfortable for you and can reduce circulation to your baby.

Back sleeping can make blood pressure drop, causing some expectant moms to experience dizziness. On the other hand, in some moms-to-be it can make blood pressure go up.

http://www.welcomebabyhome.com/pregnancy/sleeping_positions_pregnancy.htm

Occasionally you may revert to sleeping on your back or tummy because you are just not used to your new position. No harm done here. Both positions bring no harm to the fetus; the fetus is well cushioned to suffer from any pressure on the maternal body