Influence of exogenous zeitgebers may be overstated

• Laughton Miles et al (1977) recount the story of a young man blind from birth with a circadian rhythm of 24.9 hours
• despite exposure to social cues, his sleep/wake cycle could not be adjusted and consequently he had to take sedatives at night and stimulants in the morning to keep pace with the 24 hour world
• similarly studies of individuals who live in Arctic regions (where the sun does not set during the summer months) show normal sleep patterns despite the prolonged exposure to light
• both these examples suggest that they are occasions when exogenous zeitgebers may have little bearing on our internal rhythm