In some examples, a medical device delivers electrical stimulation to the external portion of the globus pallidus of a brain of a patient in order to treat a sleep impairment of the patient. In some examples, the electrical stimulation may be delivered via one or more electrodes implanted in the GPe of the brain. In some examples, an electrical stimulation device is configured to deliver electrical stimulation therapy to the GPe based on detection of a sleep state of a patient. The sleep state may include, for example, a state in which the patient is awake and intending on sleeping, is awake and attempting to sleep or has initiated sleep. In addition, in some examples, an electrical stimulation device is configured to deliver electrical stimulation therapy to the GPe based on detection of an awake state of a patient.