Poor control in studies

• Although the participants in the studies were deprived of natural light they still had access to artificial light
• for example, Siffre turned on a lamp every time he woke up which remained on until he went to bed
• It was assumed by him and others that artificial light unlike daylight would have no effect on the free running biological rhythm
• however in test Charles Czeisler et al (1999) were able to adjust participants’ circadian rhythms from 22 to 28 hours using dim lighting
• as such the use of a light may be analogous to participants taking a drug that resets their biological clock