Endocrine Abstracts

Appropriate biological timing is essential for optimal physiological function. Driven by the central pacemaker, circadian rhythms in physiology and behaviour require time cues, primarily light, to maintain synchrony with the 24h day and adapt slowly to abrupt changes of time cues. Extrinsic rhythm disorder results from the desynchrony of the internal clock with a new behavioural and/or light dark schedule. The numerous health problems of shift workers (poor sleep, alertness, performance, GI function, abnormal metabolic responses, increased accident rate and risk of heart disease, possibly greater risk of certain cancers and infertility) testify to the importance of a correctly phased internal clock. Time zone travel has similar acute problems which resolve on adaptation, but may also carry some long term risks. Intrinsic circadian rhythm abnormalities are associated with extremes of internal periodicity (tau), e.g. Familial Advanced Sleep Phase Syndrome (ASPS), possibly some Delayed Sleep Phase Syndrome (DSPS), and disorders of entrainment e.g. blindness (Non-24h Sleep Wake Disorder), possibly some DSPS. A genetic component to ASPS, DSPS and behavioural preference (earlier or later entrained phase: 'Larks' or 'Owls') has been identified. Sleep disorder associated with disturbed circadian rhythms is widespread and may often be unrecognised. Therapeutic interventions (for example timed exposure to light of suitable intensity and wavelength and/or melatonin) require a prior knowledge of internal phase to be really effective. Endogenous melatonin is the rhythm marker of choice for the determination of tau and phase in sighted and blind individuals. The urinary metabolite 6-sulphatoxymelatonin provides a non-invasive approach to the determination of rhythm parameters and is particularly useful in field situations such as the assessment of adaptation to shiftwork. Multiple types of shift schedules, environments and individual variations in response to change of time cues mean that each requires careful evaluation.