More than 70 percent of American adults sleep on their side. Many side sleepers end up sleeping with their arm under their pillow because it feels comfortable. Unfortunately, sleeping with your arm under your pillow can cause pain and numbness in your arm and shoulder. Some side sleepers may like sleeping with their arm under their pillow because it gives more support. However, it doesn’t provide the right support and may create pressure points in your neck. If you feel that your arm gives you more support than your pillow, you may actually need a new pillow. Pillows should provide plenty of cushion and support on their own. If you need your arm for more support, your pillow may be too flat or worn out. For side sleepers, you want a high loft pillow to cushion your head and cradle your neck.