In mice, the hunger hormone ghrelin (and its synthetic cousin GHRP‑6) can shift the brain's internal clock, but only when the animals have been without food for a while. Giving ghrelin to well‑fed mice didn’t change their daily activity patterns, while a single dose after 30 hours of fasting nudged the clock forward by a few hours.

The orexigenic peptide ghrelin stimulates both food intake and growth hormone release and is synthesized in the stomach and in hypothalamic areas involved in feeding control. The suprachiasmatic nuclei of the hypothalamus (SCN) control most circadian rhythms, although there is evidence that some oscillators, such as food-entrainable oscillators, can drive activity rhythms even after SCN ablation. Ghrelin levels exhibit a circadian rhythm and closely follow feeding schedules, making this peptide a putative candidate for food-related entraining signals. We examined the response of the SCN to ghrelin treatments in vitro, by means of electrophysiological and bioluminescence recordings, and in vivo, by assessing effects on the phase of locomotor activity rhythms. Ghrelin applied at circadian time 6 in vitro to cultured SCN slices induced an approximately 3 h phase advance. In addition, ghrelin phase advanced the rhythm of PER2::LUC (Period2::Luciferase) expression in cultured SCN explants from mPer2(Luc) transgenic mice. In vivo, intraperitoneal administration of ghrelin or a synthetic analog, growth hormone-releasing protein-6 (GHRP-6), to ad libitum fed animals failed to alter circadian phase. When injected after 30 h of food deprivation, GHRP-6 induced a phase advance compared with saline-injected animals. These results indicate that ghrelin may play a role in the circadian system by exerting a direct action on the SCN and that the system as a whole may become sensitive to ghrelin and other feeding-related neuropeptides under conditions of food restriction.