Fatigue is often confused with tiredness. Tiredness happens to everyone. It's a feeling you expect after certain activities or at the end of the day. Usually, you know why you are tired and a good night's sleep solves the problem.
Fatigue is dramatically different from tiredness. It is a daily lack of energy. It is an unusual or excessive whole-body tiredness not relieved by sleep. It can last just for a short time (a month or less) or stay around for longer (one to six months or longer). Fatigue can prevent you from functioning normally and gets in the way of things you enjoy or need to do.
Cancer-related fatigue is one of the most common side effects of cancer and its treatment and may persist long after treatment is complete. It is not predictable by tumor type, treatment, or stage of illness. The underlying causes of cancer-related fatigue are not well known but it is likely due to a variety of different aspects of cancer and associated treatments. Usually, it comes on suddenly, does not result from activity or exertion, and is not relieved by rest or sleep. It is often described as "paralyzing" because it can significantly interfere with daily activities.