CIRCADIAN AND ULTRADIAN VARIATIONS OF LEPTIN IN NORMAL MAN UNDER CONTINUOUS ENTERAL NUTRITION - RELATIONSHIP TO SLEEP AND BODY-TEMPERATURE

To determine the influence of circadian rhythmicity and sleep on the 2 4-h leptin diurnal variations, plasma leptin levels were measured at 1 0-min intervals over 24 h in seven normal subjects, once during noctur nal sleep, and once after an 8-h shift of sleep. The subjects were sub mitted to constant conditions (continuous enteral nutrition and bed re st in controlled chambers). Body temperature and plasma glucose and in sulin levels were measured simultaneously. During nighttime sleep, lep tin levels increased to a maximum (109.9 +/- 2.5% of the 24-h mean) an d then decreased to reach a nadir in the late afternoon. The mean diur nal variation was 18.0 +/- 3.8% of the 24-h mean. In the daytime sleep condition, leptin levels rose during the night of deprivation to a ma ximum of 104.7 +/- 2.3% of the 24-h mean, decreased to a minimum aroun d 0700 h, and then rose again during diurnal sleep (108.4 +/- 3.1% of the 24-h mean); the mean diurnal variation was 13.4 +/- 3.6% of the 24 -h mean. ANOVA revealed a significant interaction between time of day and sleep effects (P < 0.05). The diurnal and the sleep-related variat ions of plasma leptin mirrored those of body temperature and roughly p aralleled those of plasma glucose and insulin; the amplitudes of the d iurnal leptin variations were significantly correlated with the amplit udes of the diurnal body temperature variations (P < 0.05). Plasma lep tin levels also displayed irregular pulses of low amplitude (mean dura tion, 70 min) that were not affected by sleep, but were associated wit h a significant decrease in glucose and insulin levels (P < 0.01). The se results demonstrate that under continuous enteral nutrition, plasma leptin levels are modulated by both a slight circadian component and sleep, which interact under normal conditions, and suggest that leptin is implicated in circadian thermoregulatory adjustments.