GROWTH-HORMONE SECRETION IN NIGHT WORKERS

We previously reported that, in night workers, cortisol and TSH rhythm s, known to have a high endogenous component, adapted only partially t o the nocturnal schedule. The aim of the present study was to investig ate the degree of adaptation of the growth hormone (GH) rhythm, consid ered to be mainly sleep-dependent, but for which a weak circadian driv e has also been suggested. Eleven night workers were studied during th eir usual sleep-wake cycle, and two groups of 11 normally day-active s ubjects, sleeping once during the night and once after an g-h sleep de lay, were used as control groups. GH secretory rates were calculated b y deconvolution of the plasma concentrations analyzed at 10-min interv als. The total amount of GR secreted during the 24 h did not differ be tween the three groups and the main secretory episode occurred, in mos t cases, during the first half of the sleep period. In night sleepers and night workers the enhanced amount of GH secreted at that time was followed by a significantly lower amount secreted during the second pa rt of the sleep period (p < 0.001 and p < 0.05, respectively). For nig ht sleepers, an enhanced GH pulse frequency was found at the beginning of sleep, whereas for night workers and day sleepers the pulses were distributed more randomly throughout the nychthemeron. After an abrupt sleep shift, all the subjects displayed a GH pulse at the usual time of early sleep, but such a pulse was present in only 8 of 11 night wor kers. Thus the amount of GH secreted between 23:00 h and 03:00 h in da y sleepers did not differ significantly from that observed in night sl eepers, whereas it differed for night workers. These results confirm t he considerable influence of sleep in driving the GH rhythm and the ex istence of a circadian influence revealed by an acute shift in the sle ep period. They also provide evidence of an incomplete adjustment of G H rhythms in night workers.