Temporal changes in growth hormone, cortisol, and glucose: relation to light onset and behavior

Plasma growth hormone (GH), cortisol, and glucose were measured every 20 min in the last 2 h of a 12 h dark cycle and the first 4 h of a 12 h light cycle in eight environmentally limited rhesus monkeys; additionally, ongoing behaviour was scored every 20 min in the last 4 h when the lights in the booths housing the monkeys were on. Episodic fluctuations in levels of GH, cortisol, and glucose occurred. Onset of booth illumination was associated with a significant number of GH secretory bursts (6 of 18), and more than twice as many bursts per hour occurred in the light than in the dark (L:D = 0.44:0.19). The onset of illumination was also associated with significant increases in glucose for the eight animals. In addition, plasma glucose increased concurrently with the onset of 13 of 18 bursts of GH secretion. Monkeys were rated as significantly more alert or aroused when plasma levels of GH and glucose were increasing than when they were decreasing. In contrast plasma cortisol showed small, rhythmic fluctuations over time that did not correlate with booth illumination or degree of behavioral arousal. This latter finding challenges the traditional assumption that cortisol is a sensitive index of behavioural arousal.