To determine whether the ultradian and circadian rhythms of glucose and insulin secretion rate (ISR) are adapted to their permanent nocturnal schedule, eight night workers were studied during their usual 24-h cycle with continuous enteral nutrition and a 10-min blood sampling procedure and were compared with 8 day-active subjects studied once with nocturnal sleep and once with an acute 8-h-shifted sleep. The mean 24-h glucose and ISR levels were similar in the three experiments. The duration and the number of the ultradian oscillations were influenced neither by the time of day nor by the sleep condition or its shift, but their mean amplitude increased during sleep whenever it occurred. In day-active subjects, glucose and ISR levels were high during nighttime sleep and then decreased to a minimum in the afternoon. After the acute sleep shift, the glucose and ISR rhythms were split in a biphasic pattern with a slight increase during the night of deprivation and another during daytime sleep. In night workers, the glucose and ISR peak levels exhibited an 8-h shift in accordance with the sleep shift, but the onset of the glucose rise underwent a shift of only 6 h and the sleep-related amplification of the glucose and ISR oscillations did not occur simultaneously. These results demonstrate that despite a predominant influence of sleep, the 24-h glucose and ISR rhythms are only partially adapted in permanent night workers.