Position: Side with Arm Under Head
Affects neck, shoulders, arms, fingers
It may feel comfy, but resting your head (which weighs, on average, 10 pounds) on your arm for hours at a time stresses the body.

"This position presses on the nerves that run through the top of the shoulder and down the arm, resulting in numbness in your fingers," FitzGordon explains. The shoulder you sleep on also gets hiked up toward your ear, which constricts the muscles of the shoulder and neck.

Pillow talk 
If you're a side sleeper, look for a pillow that fills the space between your ear and the outer edge of your shoulder when you're lying on your side. "Your pillow should enable both sides of your neck to be equally long," FitzGordon says, which promotes easier breathing and prevents overstretching on one side of your neck. Any pillow that maintains its shape (foam, for instance) is fine. To keep you from hiking one leg up, which rotates the pelvis and contorts the lower spine, Kirsch suggests placing a pillow that is 6 to 8 inches high between your knees.

Fix: shoulder stretch
What it does: Stretches the back of the shoulders, which tend to get compressed by the weight of your head.

How to do it: Sit tall, core tight, and extend your left arm straight out to your side at shoulder height, palm facing down. Now reach that arm across your chest and use your right hand to draw it in closer to you. Relax for five breaths, then switch arms. Stretch each side three times.

Fix: cow face pose
What it does: Stretches and brings balance to the muscles of the neck as well as the shoulders. Opens the chest.

How to do it: Hold a strap in your right hand. Standing with your core tight, raise your right arm straight up and turn your hand so the palm faces behind you. Now bend your elbow and bring your hand as far down your back as you can. Reach the left arm straight out to your side, then turn your palm to face behind you. Bend your left elbow and bring your left arm behind your back; grab the strap with your left hand. Relax for five breaths. Repeat on other side.