Therapy delivery to a patient may be controlled based on a determined sleep stage of the patient. In examples, the sleep stage may be determined based on a frequency characteristic of a biosignal indicative of brain activity of the patient. A frequency characteristic may include, for example, a power level within one or more frequency bands of the biosignal, a ratio of the power level in two or more frequency bands, or a pattern in the power level of one or more frequency bands over time. A therapy program may be selected or modified based on the sleep stage determination. Therapy may be delivered during the sleep stage according to the selected or modified therapy program. In some examples, therapy delivery may be controlled after making separate determinations of a sleep stage based on the biosignal and another physiological parameter, and confirming that the sleep stage determinations are consistent.