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.