Many conditions can cause pain and fatigue that lead to the need to lie down and rest frequently throughout the day. Those conditions include chronic back pain, heart disease, fibromyalgia, lupus, and cancer. If you need to rest in a recumbent position more than an hour out of the working portion of the work day, in other words for more than the typical lunch and two breaks, then you could be considered disabled for that reason. Employers expect you to be at your work station. If you cannot do that, you will not be employed for long. Whether or not you are suffering from fatigue or chronic pain, your need to rest lying down will seriously affect your ability to work.