The use of constant routines in unmasking the endogenous component of human circadian rhythms.

A major problem in the study of the internal clock(s) that drives human circadian rhythms is that due to the effect produced by rhythmicity of habits and external influences ('masking'). A particularly potent factor in this respect is the sleep-wake cycle. It is anomalous that, even though this masking influence is widely accepted, most studies of circadian rhythmicity have been performed in the presence of such interferences. A protocol is described, the constant routine, by which these exogenous influences can be minimized, thereby enabling a closer scrutiny of the internal clock(s) to be made. An account is given of the different circumstances in which the constant routines have been used together with the results derived from such studies. Briefly, they indicate that nychthemeral studies can give misleading information about the rate of adjustment of the internal clock to various manipulations, e.g. time-zone transition, shift work. In addition, future studies making use of constant routines are described, in particular those which might enable the presence of more than one internal clock to be established.