According to The Huffington Post, a study examined four sleep positions in adults without sleep apnea to see which would best help a person rest.
In a study conducted by the Japanese Society of Sleep Research, investigators looked at four different sleep positions in adults without apparent obstructive sleep apnea: right, left, prone (on your stomach), and supine (on your back). Even though we all have our own creative modifications, we can generally relate to these four distinct directions.
Among the 20-40 age group, researchers did not find a dominant position. However, they did find that females slept longest in the supine (on your back) position and males slept longest in the right position.