This has apparently helped me!

[Sailor]

Gentlemen;
I've been having this problem for years. I realize symptoms come and go and severity varies enormously over time. However, during a particularly bad period about 3 months ago, I tried something that seems to have reduced my symptoms enormously. I won't bore you with the reasoning and experiences that led me to try this approach, but I can say that I felt much better after a week, and, after 3 months my symptoms have been reduced considerably (but not eliminated). I still have periods when symptoms flare but I get them under control much quicker now.
The change in my life that seems to help is forcing myself to sleep on my back with a pillow under my knees. Seems crazy, but it helps. All my life I have been a side-sleeper so sleeping on my back did not come easy. However, after sleeping on my back with knees lifted for a couple of nights, my symptoms eased and have continued to be much reduced. I still have periods when symptoms flare, but invariably, a night's sleep on my back helps much. Who knows, maybe sleeping in that position helps to reduce tension in those muscles.
Anyway, thought I would post this since perhaps it might actually help.

[MC]

That seems very possible depending on which muscles are causing your pain. For me I can't sleep on my side unless I scissor my legs very wide. If I keep my legs together while laying on my side I wake up very tight in the perineum area. I did notice that when I sleep on my chaise (a long cushon on the end of my sectional couch) with my head and lower back on the back of the couch so I'm in a slouched position with my feet up on the chaise I rarely awake with any pain. So possibly something about keeping our bodies semi folded is allevaiting some muscle tension. It makes sense. I wonder how it would be sleeping on one of the special beds you see on the infomercials. The ones that fold up that older adults often buy, hmm.

[webslave]

The orginal poster could be relieving a nerve problem in his spine by lying on his back with legs raised. It is possible, I believe, for lumbar spine problems to lead to pelvic inflammation via cross-talk and other neurological phenomena.

[winding_road]

maybe sleeping in that position helps to reduce tension in those muscles.
Anyway, thought I would post this since perhaps it might actually help

That's maybe possible . Do you have trigger points in your Abdomen ?
to release the tension in those muscles , My PT suggest the same position as you describe.

[iHurt]

I've been sleeping with a backrest pillow under my knees since 1987 after having back surgery due to a car accident. My orthopedic surgeon said it removes all tension from your lower back and lower abdomen so yeah it does the trick for me too. I typically feel like a million bucks in the morning.

If you're a side sleeper, try keeping your knees bent at a 45 degree angle and wedge that pillow between your legs so your knees are a 4-8 inches apart. You'll feel those muscles relax right away. I sleep in both positions and could never sleep without it now!

Cheers,