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Sigma-rhythm (σ-rhythm, sleepy spindles, spindle activity, explosive or flare activity, spindle-shaped flash, the α-similar rhythm of animals, barbituric spindles) is one of the most distinct elements of spontaneous EEG recorded in a state of natural sleep. Occurs in some neurosurgical and pharmacological effects. First described by Loomis and co-authors (Loomis et al., 1935) in the study of EEG expression of different stages of sleep.

Spontaneous Sigma-rhythm has a frequency of from 10 to 16 Hz, but mostly at 12-14 Hz. The characteristic feature of the Sigma-rhythm is the increase in the amplitude at the beginning of the outbreak of the Sigma-rhythm and its descending at the end of the outbreak. The amplitude is different, but in adults is mainly not less than 50 µv.

Sigma-rhythm appears in the initial stage of NREM sleep that directly follows the NAP. During sleep, Delta waves Sigma-rhythm is rare. In the process of transition to fast sleep Sigma-rhythm observed in EEG, but completely blocked in the developed REM sleep.

Of vertebrates, Sigma-rhythm was only in mammals, with the exception of the detachment of the monotremes (echidna). Sigma rhythm is absent in fish, amphibians, reptiles and birds. With regard to ontogenesis, it describes the different timing of the appearance of the Sigma rhythm. In rodents it appears at 10-15 days after birth. Kittens and puppies Sigma-rhythm occurs later. In humans this rhythm arises from about the age of three. With age, the frequency of oscillation rhythm, as a rule, does not change.