Being tired is often just a part of life. If you work hard, spend enough time at the gym, and enjoy your life, you'll find yourself falling into bed exhausted every night. However, there are times when the fatigue is much stronger than it should be. You have a hard time getting out of bed in the morning, even after a solid 8 or 9 hours of sleep. You can't bring yourself to go to the gym or get through your workout, and you feel the tiredness all the way to the bone. [caption id="" align="aligncenter" width="610"] Source: fatiguescience.com[/caption] There are many disorders that can cause serious fatigue, including autoimmune disorders like rheumatoid arthritis and Celiac disease. However, a number of different types of cancer also can be the cause of your fatigue. Cancers that affect your endocrine system are known to cause profound weariness, even after you've spent the last few days or weeks resting. If you feel like you never have any energy, it's time to get checked out!