Somnolence is an early manifestation of a disorder of consciousness. Narcolepsy is associated with different symptoms or signs, and the corresponding possible pathologies are different. Patients with narcolepsy show excessive prolonged sleep, but can be awakened and can barely cooperate with examinations and answer simple questions after waking up, and after stopping stimulation the patient goes back to sleep. In pure narcolepsy, the possibility of narcolepsy should be considered. If headache is present, the possible lesions are encephalitis, meningitis, subarachnoid hemorrhage, and traumatic brain injury. With hemiparesis, the possible lesions are cerebral infarction, cerebral hemorrhage, and traumatic brain injury. With fever, the possible lesions are encephalitis, meningitis, and sepsis. With bradycardia, the possible lesions are hypothyroidism and heart disease. In addition, patients with metabolic diseases such as diabetes mellitus and electrolyte disorders may also show drowsiness and should not be ignored. Narcolepsy is a manifestation of decreased wakefulness of consciousness disorder, and the maintenance of consciousness depends on the excitation of the cerebral cortex. The state of wakefulness is maintained by the superior brainstem reticular activation system and the bilateral cerebral cortex. Therefore, from the perspective of neurological conduction integrity, impairment of either of these two can cause narcolepsy. Thus, somnolence can be a precursor to a variety of diseases. In case of significant somnolence, a hospital should be consulted promptly for a CT or MRI of the head to identify the specific cause to avoid the presence of serious brain lesions that could be life-threatening.