I'm trying to correct my anterior pelvic tilt (APT) which is a result of bad sitting posture / prolonged sitting. Because our body adapts to the positions in which we fix our muscles, and one period during which we tend to keep our muscles in some fixed position for an extended period of time is sleep (assuming at least an hour or two, because people do move in their sleep), I wanted to make sure I was addressing my sleeping posture.
For a long time, I have been sleeping on my side, but recently, I saw a video suggesting that sleeping on one's back is helpful for correcting APT. According to the video, when in this position, it is important to place pillows under one's knees to allow the pelvis to posteriorly tilt which will relieve the feeling of tightness in the lower back due to extension of the lumbar spine. This tightness / extension without pillows beneath the knees is a result of shortening of the iliopsoas, which is shortened in people with APT.
I'm confused because if the iliopsoas is shortened, then, to correct the resultant APT, wouldn't we want to stretch the iliopsoas and hip flexors as shown here (and up to 30 seconds before the start of the link)? And since sleeping with a pillow under the knees causes shortening of the iliopsoas, wouldn't this exacerbate the tilt?