EFFECTS OF THE ENDOGENOUS CLOCK AND SLEEP TIME ON MELATONIN, INSULIN,GLUCOSE AND LIPID-METABOLISM

This study was undertaken to determine whether the internal clock cont ributes to the hormone and metabolic responses following food, in an e xperiment designed to dissociate internal clock effects h-om other fac tors. Nine female subjects participated. They lived indoors for 31 day s with normal time cues, including the natural light: darkness cycle. For 7 days they retired to bed from 0000 h to 0800 h. They then underw ent a 26-h 'constant routine' (CR) starting at 0800 h, being seated aw ake in dim Light with hourly 88 Kcal drinks. They then Lived on an imp osed 27-h day (18 h of wakefulness, 9 h allowed for sleep), for a tota l of 27 days. A second 26-h CR, starting at 2200 hi was completed. Dur ing each CR salivary melatonin and plasma glucose, triacylglycerol (TA G), non-essential fatty acids (NEFA), insulin, gastric inhibitory pept ide (GIP) and glucagon-like peptide-1 (GLP-1) were measured hourly. Me latonin and body temperature data indicated no shift in the endogenous clock during the 27-h imposed schedule. Postprandial NEFA, GIP and GL P-1 showed no consistent effects. Glucose, TAG and insulin increased d uring the night in the first CR. There was a significant effect of bot h the endogenous clock and sleep for glucose and TAG, but not for insu lin. These findings may be relevant to the known increased risk of car diovascular disease amongst shift workers.