We seek a methodological approach to assess circadian processes in subjects who have recently experienced traumatic brain injury, using regularly gathered intracranial temperature data. The health effects of circadian regulation are profound, yet assessments of circadian processes are often infeasible in the neurotrauma intensive care unit (ICU). We assess circadian periodicity through the energy density spectrum, applying model-based and model-free methods: an extended Kalman filter (using the Kronauer model) in conjunction with a fast Fourier transform (FFT), and the Lomb–Scargle periodogram, respectively. We compute the change in relative energy in a narrow band of the spectrum, corresponding to a period of 23-25 h, over the first 48 and last 48 h of each subject's stay in the ICU. The change in relative energy in the spectrum increases in subjects with Good outcomes, and decreases in subjects with Poor outcomes, with statistical significance. For those subjects with Good outcome, we show evidence of regulation in circadian phase using an adaptive notch filter.