I really struggled to get comfy sat right upright. I managed it but by day 6/7 I was exhausted from broken sleep and my back and neck were in bits. I’m 10days po today and I spoke to my nurse at my day 7 check and she just said look everyone struggles. Nothing bad will happen if you lie flat on your back, but upright helps reduce swelling and encourages the implants to drop and settle. I’m now pretty much sleeping on my back. Maybe just a slight angle. I think actually getting sleep is gonna be more important for me right now. I really wouldn’t worry at all. Try your best to start the night upright but as long as your only waking on your back and not your front or sides you won’t do them any harm.