One of the mysteries of the English language finally explained.
1no object Of a signal generator, timepiece, etc.: to operate independently, without external influence or intervention. Also in extended use: to run freely, loosely, or easily.
2no object Biology. Of a circadian rhythm: to operate in the absence of, or independently of, light or other external cues indicating the time of day. Also (of an organism): to maintain a circadian rhythm in this way.
1940s. From free + run.
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