As shown here and here, electroshock in the USA is commonly done with the patient reclining or flat on their back as on a gurney. The posture is usually symmetric. The actual position used is a possible imprint, and becomes a likely imprint after subsequent electroshock sessions. After electroshock, a similar position is often used in the recovery room as the anaesthetic wears off and the patient awakens (they usually just roll you from a treatment area to a larger recovery room).
When the patient goes home after treatment they may experience restless sleep or notice that other sleep positions (not reclined or not flat on their back; other than that used in electroshock) seem preferable. So as the weeks and months pass; upon going to bed, they notice gradually being drawn to side sleeping or tummy sleeping or sleeping with a bent knee. Any asymmetric sleeping posture may be helpful. For example those who were treated while flat may sleep better in a recliner than in a bed.