Used in the parlance of circadian rhythms, it is the naturally occurring period of a biological rhythm as it exists in constant conditions. Everyone has a genetically determined day-night cycle that is influenced by external conditions such as exposure to light. When these external conditions are controlled, the individual begins "free-running," which reveals their independent circadian period. This is typically not quite 24 hours in length and may be slightly shorter or longer. It becomes important in blind people, as without light perception they will experience cyclical insomnia and hypersomnia every few weeks.