Infralow frequencies and ultradian rhythms.

Our brain activity demonstrates amazing stability across multiple time frames ranging from a few milliseconds to several hours. The longer cycles are commonly called ultradian rhythms and they correspond to infralow frequencies (ILFs) in the milli-Hz range (0.001 Hz). Ultradian rhythms between 90 minutes and 2 hours or longer are readily observed in our electroencephalogram, and they reflect periods of activity and rest, cycles of cortical excitability and plasticity followed by relative inactivity. Our nightly sleep is organized into similar stages (rapid eye movement and non-rapid eye movement sleep) as is our daily behavior (ie, the basic rest-activity cycle). Astrocytes often exhibit milli-Hz ILFs, and they play a major role in shaping neuronal plasticity and activity, and thus may organize or influence the basic rhythms of sleep and waking. The nature and importance of astrocytes in human brain functioning is subsequently reviewed.