Definition: Free running is a term used to describe a circadian rhythm that is not entrained to any kind of external time cues, such as the natural dark-light cycle.
Individuals tend to persist at their endogenous (or internal) day-length, generally slightly longer than 24 hours, and drift across the actual 24-hour day. They tend to have a sleep-wake cycle that is asynchronous, leading to inappropriate insomnia and hypersomnia as their sleep propensity is misaligned to nighttime.
Free running may occur in the blind or those who are kept in constant low-light conditions.
Examples: When normal people are placed in constant darkness, their circadian rhythms will be free running without the influence of the dark-light cycle.
