Feeling tired despite sleeping can come from sleep that is broken or non-restorative — for example, from sleep apnea, stress, caffeine or alcohol late in the day, an irregular schedule, or screen use at night. Lifestyle factors such as dehydration, low physical activity, skipping meals, and low mood or anxiety are also very common contributors and are often reversible.
Medical causes worth ruling out include iron-deficiency anemia, an underactive thyroid, diabetes, vitamin deficiencies (such as B12 or vitamin D), infections, and side effects of some medicines. If fatigue is severe, lasts more than a few weeks, or stops you doing normal activities, see a doctor, who may suggest simple blood tests.
Seek prompt care if fatigue comes with chest pain, breathlessness, unexplained weight loss, fainting, or very heavy periods. A clinician can interpret your symptoms in context. This is general information, not a diagnosis.