If all brain related causes (transient ischemic attack, disorders of hypothalamus, dementia and multiple sclerosis, head injury etc) are ruled out then Vit B12 deficiency, chronic fatigue syndrome, sleep disorders
(by sleep studies), depression, use of
certain drugs, and certain types of
seizures (where memory loss is the predominant symptom) should be investigated for as the cause of memory loss. Apart from sleep studies, EEG, you may need cognitive tests. Please consult a neuropsychiatrist.