Yes, it's totally fine! When it becomes uncomfortable for you (3rd trimester for me last pregnancy), then you stop. The concern is that the weight of the baby will compress the inferior vena cava, reducing blood flow to the baby. Baby is too small now to do that, and once baby gets that big, it affects you before it affects baby. If you're getting lightheaded sleeping on your back, turn over.
tbh, I think it's usually okay for a bit, but just listen to your body and switch it up if it feels uncomfortable!
I actually checked the 'Heal Baby Care App' for this and here's what it says:
"Sleeping on your back for a short period during early pregnancy, like at 11 weeks, is generally considered safe. However, as the pregnancy progresses, it’s recommended to switch to your side, particularly the left side, to improve blood flow to the baby and reduce pressure on your organs. If you’re comfortable, you can use pillows for support. Are you experiencing any discomfort or other concerns while sleeping?"
Re: Sleeping on back ok for first trimester?