Rhythmic feet movements while falling asleep.

During the wake-sleep transition and sleep, diverse motor phenomena such as hypnagogic foot tremor may occur in the lower extremities. We investigated the relevance of this phenomenon in 375 consecutive subjects examined polysomnographically in a sleep disorders center. Rhythmic feet movements while falling asleep (RFM) were found in 28 subjects (7.5%). RFM occurred mostly as single, short series with a duration of between 10 and 15 seconds. They had a high night-to-night variability and were detected as rhythmic, oscillating movements of the whole foot or toes. Surface electromyographic (EMG) recordings displayed series of repetitive phasic bursts with a periodicity mostly between 1 and 2 per second. Single EMG burst duration varied between 300 and 700 msec. RFM at highest intensity occurred during presleep wakefulness, and usually persisted in sleep stages 1 and 2. RFM did not have a major sleep-disturbing effect in any of the affected subjects. Due to its high prevalence and the lack of a major sleep-disturbing effect, short series of RFM could be considered a quasiphysiological phenomenon. However, in more severe forms of RFM with evidence of a sleep-disturbing effect, RFM should be considered abnormal.