About once per week, I see mostly female patients who come in for recurrent sinus or throat problems who also have severe and chronic fatigue. When asked if they sleep on their backs, they'll say yes. But when I question them further, they'll tell me that when they were younger, they always slept on their stomachs, with their face on one side of the other. Then I asked about when they began to sleep on their backs, and not too surprisingly, it's about the same time that they began to feel more tired and started to have various illness such as sinus infections, throat pain, etc.
When asked why they began to sleep on their backs, the most common answer usually is, "my dermatologist said sleeping on my face could aggravate wrinkles" (read the article from the American Academy of Dermatology here). What the dermatologist does not appreciate, however, is that these people MUST sleep on their stomachs so they can breathe well when sleeping at night. The reason for this is that many people (to various degrees) have a tendency for their tongues to fall back slightly when lying on their backs due to gravity. When you add deep sleep, all the muscles begin to relax, and the tongue may collapse completely, which causes a temporary obstruction and arousal. This prevents people from getting deep sleep. This is what I talk about in my book, Sleep, Interrupted.
If you have this condition, you probably realized this subconsciously when much younger and slept on your side or stomach to compensate pretty well. But when you start to sleep on your back, then you can't compensate very well anymore and you will have multiple micro-obstructions and arousals, preventing you from achieving restorative, deep sleep. So in a sense, this will age you more in the following manner: inefficient sleep causes a low grade stress response, constricting blood vessels to nonessential organs such as your gastrointestinal or reproductive organs, skin and hands or feet. If you don't get enough blood flow. your skin cannot heal and repair itself properly, this "aging" faster. Plus you also feel tired and lousy.
This situation can also apply to people who are admitted to the hospital after operations or after an accident, but in these situations, the consequences can be much more severe. Others have to sleep on their backs due to an shoulder injury or neck pain, which prevents stomach sided sleeping. Some people ABSOLUTELY cannot sleep on their backs. Something to think about.