An object of the present invention is to provide a sleep determination device and a sleep determination method capable of precisely determining a sleep stage based on heartbeat information of a subject. The sleep determination device and the sleep determination method according to the present invention obtain the heartbeat of the subject, calculate a heartbeat cycle for each heartbeat based on the obtained heartbeat, calculate difference values until obtaining Nth-degree difference values set in advance with difference values between the successive heartbeat cycles as first-degree difference values and difference values between the successive first-degree difference values as second-degree difference values, generate a return map and a histogram indicative of a distribution of the respective values in a predetermined period of time for each of the heartbeat cycles and the difference values from the first-degree difference values to the Nth-degree difference values, and determine a sleep stage of the subject based on the generated return map and the histogram by referring to a return map and a histogram for determination set in advance for each sleep stage.