(HealthDay News) -- Chronic fatigue syndrome is characterized by extreme tiredness and a feeling of being worn out all the time, even after waking up. Its cause isn't understood.

The U.S. National Women's Health Information Center says chronic fatigue syndrome may have these symptoms:

• Extreme tiredness for longer than 24 hours, especially after exercise.
• Forgetfulness, or difficulty maintaining focus.
• Tiredness that persists after sleep.
• Muscular aches and pains.
• Joint pain that isn't accompanied by redness or swelling.
• Headaches that vary in severity, pattern or type.
• Soreness of the lymph nodes beneath the arm or in the neck.
• Sore throat.