Endogenous Circadian Period Estimation (Lomb-Scargle)

Estimates the dominant (endogenous) circadian PERIOD (tau) of an activity time series using the Lomb-Scargle periodogram. Unlike the classical Fast Fourier Transform (FFT), the Lomb-Scargle method does not require evenly sampled data, so it correctly handles the irregular and gappy sampling that results from non-wear periods, dropped epochs, or mixed epoch lengths in accelerometer recordings.