Brainstem origin for a new very slow (1mHz) oscillation in the human non-REM sleep episode.

The time-courses of power in the different frequency bands (1-40 Hz) within the non-rapid-eye-movement (NREM) episode of the human sleep electroencephalogram have provided for many years a fascinating window into the sleep process. Here our analysis of the slow-wave band (1-4 Hz) reveals a hitherto unrecognized very slow oscillation of power with mean period ~15 minutes, an instability that appears to be an integral characteristic of the early NREM episode. The neuronal transition probability (NTP) model has already given a mechanism explaining how power in the spindle band peaks consistently before that of slow wave activity. Here we show that an extension of the model, with the hypothesis of a population of sleep neurons alternating between two steady probability states, can simulate the very slow oscillation. In doing so it gives not only the time course of power in the slow wave band, but also the simultaneous time-courses in the spindle and in the fast frequency bands. Animal data suggest that a brainstem neuronal population, toggled by an external switching source, generates these time-courses and dictates them to the thalamus and thence to the cortex. The discovery of the very slow oscillation and the success of the NTP model in interpreting the overall NREM structure may have important implications for both clinical and fundamental sleep research.