On a side, with the head tilted over the edge of the mattress, with the pillow over the head, completely under the blanket or without a blanket at all, it all depends on what makes you feel comfy and leads you to sleep. The room temperature is a great factor in all this as is the person you share your bed with.
- the soldier position is prone to snoring and waking up with dry mouth
- you get more oxygen when you sleep sitting but it fees uncomfortable and you can wake up with neck pain
- a pillow between your knees makes sleeping on a side way more comfier