BEST: Lying on your back.
Whether your arms are to the side (the "Soldier" position) or up (the "Starfish"), lying on your back is the best position for your spine, making it the best for your whole body too. It keeps and promotes the natural alignment of your head, your neck, and your back.
It also prevents you from getting acid reflux. Lying on your back means your head is elevated and your stomach is below the esophagus, so digested substances won't be going back up.
This also prevents your skin from breaking out since your face isn't in contact with your sheets, which get dirtied by the oils from your hair and body. And when was the last time you washed your pillowcase? You get the point.
Note: If you snore or if you're pregnant, this one's not for you. Lying on your back makes the base of your tongue and soft palate lean to the back wall of your throat, causing a vibrating sound (the snore) during sleep. If you're pregnant, it'll make you dizzy and it can interfere with the flow of blood and nutrients to your developing baby.
SECOND BEST: Sleeping on your side
If you snore or you're pregnant, do this. It'll clear up that part of your throat behind your mouth. Make sure your body is STRAIGHT so your spine is still elongated—this will ease back pain.
The only downside to this is what it can do to your skin (hello, acne), especially if you don't change your sheets often. Oh, and there's also skin aging, caused by the pressure or stress you put on your face, since it'll get squished on one side.
BAD: Lying on your belly
This doesn't support the natural curve of your body, and it can lead to the overarching of your spine and bad posture. It also places pressure on your joints that can lead to pain and tingling.
Lying on your belly forces your neck to be rotated in a tight position, which compromises your breathing and circulation.
However, this position helps in digestion. So if your stomach hurts, lie on your belly until the pain goes away, then shift after a few minutes.
BAD: Fetal position
When you lie on your side with your knees to your chest, you're making yourself vulnerable to neck and back pain and strains on your joints. The shoulder and neck should be aligned when you're sleeping.
What more, if you're curled up and sleeping on your left side, you put stress on your vital organs like your liver, your stomach, and your lungs.
Remember not to keep forcing yourself to sleep in a position you're not comfortable with. Sometimes the quality of your sleep is more important—and you get good night's rest when you follow your natural sleeping position. It would be wise to try sleeping in the best position for your body first and see where it'll take you.
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