Adaptation to an 8-h shift in living routine by members of a socially isolated community

The proportion of the working population employed on some form of shift system has increased substantially over the past two or three decades. Expert opinion as to the optimal form of shift system is divided, however, between those who favour ‘permanent’ systems and those who favour ‘rapidly rotating’ ones. We have studied the adaptation of half the members of a socially isolated community to a permanent 8-h shift in their routine. Even in this seemingly ideal situation we failed to find complete adaptation of the ∼ 24-h circadian rhythm in either body temperature or performance efficiency over the 10-d study period. This finding casts some doubt on the theory that permanent shift systems are optimal, and that the light–dark cycle is not important in entraining human circadian rhythms.