If the person gets too tired and overwhelmed as compared to others doing the same job, it also can be suspicious, often associated with some serious health conditions. In case feeling tired is not the sign of some mental disorder, the cause might be a stomach cancer. Waking up at night too often is not okay anyway. The symptom is worse if you are sure you had enough rest, but still feeling sleepy and exhausted. It is critical to let the doctor define the underlying condition.