A good night's sleep—everyone needs it, but not everyone gets it. We stay up late and get up early, and then wonder why we always feel tired. Why is sleep important? See how much you know about snoozing by taking this quiz.

Why is sleep important? See how much you know about snoozing by taking this quiz.

When your sleep is upset throughout the night, you can be very sleepy during the day. Maybe you have sleep apnea.

Red blood cells are usually doughnut-shaped. In a person with sickle cell anemia, the red blood cells are shaped like a C--like a sickle.

A glenoid labral tear and a torn rotator cuff are just two common injuries that can happen with the shoulder. Can you think of others? Find out how much you know about shoulders by taking this quiz, based on information from the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

The first sign of shingles is often burning or tingling pain, or sometimes numbness, in or under the skin. You may also feel ill with fever, chills, headache, or upset stomach.

Repetitive-stress injuries (RSIs) can be painful and debilitating, causing damage to tendons, ligaments, nerves, and muscles over time. Also called repetitive motion syndromes, RSIs can affect almost any part of the body. Test your knowledge of RSIs by taking this multiple-choice quiz.

True or false: If you have Raynaud's phenomenon, you will know it because of your fingers and toes change color.

Radon is a gas that you can't see or smell, but is present in nearly all the air around us. Everyone breathes in radon every day, usually at low levels. People who inhale high levels of radon are at increased risk for developing lung cancer. Radon is a problem in homes in which higher concentrations collect. Find out more about radon by taking this quiz.

Paget disease is a condition that affects the bones, usually causing them to become enlarged and deformed. Worldwide, it is the second most common bone disease. Learn more about Paget's disease by taking this quiz.