Is laying on your back during pregnancy truly harmful to the mother and/or baby?

I woke up this morning on my back. I have heard this isn’t an ideal sleeping position while pregnant, especially after the 2nd trimester. I can’t control how often I turn and move during the night. I wake up on my back a lot. Apparently it’s comfortable! Can anyone tell me why it’s so bad to sleep in that position? Will it harm my baby?