Different light-dark cycles affect growth rate and food intake of mice

The adaptation of the endogenous rhythm of an organism to external cycles may influence the development of physiological processes in animals. Light not only synchronizes the circadian system, but also exerts profound direct effects: the immediate reduction of melatonin release at night-time and the inhibition of locomotor activity in nocturnal rodents after a light pulse are well-known examples, yet little is known about effects of different light/dark (LD) cycles on the level of corticosterone, growth hormone and growth rate. Mice were raised under different period length of LD cycle including LD5:5 (light: 5 h; dark: 5 h), LD 12:12 (light: 12 h; dark: 12 h) and LD 16:16 (light: 16 h; dark: 16 h) for four weeks. Mice in LD5:5 and LD 16:16 groups manifested higher locomotor activity, plasma corticosterone and growth hormone concentrations and growth rate than the LD 12:12 group. The results suggest that different LD cycles may affect many physiological processes including growth rate, food intake and hormones, and the change of growth rate in different LD cycles may be related to the level of corticosterone and growth hormone concentrations. The results also suggest that both the long-period LD cycle and short-period LD cycles can improve the growth of mice, but they disturbed the biorhythm stabilization and affected hormone secretion; in general, these conditions would not promote the animals' survival.