Growth hormone secretion in night workers

We previously reported that, in night workers, cortisol and TSH rhythms, known to have a high endogenous component, adapted only partially to the nocturnal schedule. The aim of the present study was to investigate the degree of adaptation of the growth hormone (GH) rhythm, considered to be mainly sleep-dependent, but for which a weak circadian drive has also been suggested. Eleven night workers were studied during their usual sleep-wake cycle, and two groups of 11 normally day-active subjects, sleeping once during the night and once after an 8-h sleep delay, were used as control groups. GH secretory rates were calculated by deconvolution of the plasma concentrations analyzed at 10-min intervals. The total amount of GH secreted during the 24 h did not differ between the three groups and the main secretory episode occurred, in most 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 part of the sleep period (p < 0.001 and p < 0.05, respectively). For night 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 workers. Thus the amount of GH secreted between 23:00 h and 03:00 h in day sleepers did not differ significantly from that observed in night sleepers, whereas it differed for night workers. These results confirm the considerable influence of sleep in driving the GH rhythm and the existence of a circadian influence revealed by an acute shift in the sleep period. They also provide evidence of an incomplete adjustment of GH rhythms in night workers.