Provided are an alertness device configured such that a load is low in the processing for alertness determination and that the accuracy in alertness determination is high, a seat including the alertness device, and an alertness determination method. The alertness device includes a heart rate sensor configured to obtain an electrocardiographic signal of a person, a calculation unit configured to calculate the electrocardiographic signal obtained from the heart rate sensor, a waveform generation unit configured to generate an RRI waveform from an electrocardiographic waveform of the electrocardiographic signal, and a determination unit configured to determine the alertness of the person based on the electrocardiographic signal. The calculation unit replicates the same number of RRI waveforms as that of anchors, each anchor being a point where a certain RRI is shorter than the next preceding RRI. Then, the calculation unit moves the time axes of the replicated RRI waveforms such that the anchors of the replicated RRI waveforms are in phase with each other, and calculates a PRSA signal defined as an RRI average for each time axis. The determination unit determines the alertness of the person based on the RRIs and the PRSA signals.
