The Siffre study, and others were only conducted on a handful of people and, consequently, it is impossible to generalise these results to an entire population, as there may be individual differences in the participants which could account for their response to changes in the sleep/wake cycle. For example, Czeisler has found evidence to show cycles can vary by as much as 13-65 hours. This suggests results cannot be generalised from small samples, as factors such as age and gender may also impact our circadian cycles.