Abstract Orexins, or hypocretins, are peptides originally found in the hypothalamus, and have been shown to be involved in the stabilization and maintenance of sleep and wakefulness. In addition, these peptides are known for their actions on energy homeostasis by increased heat production or physical activity. Previous results suggest them to be also involved in peripheral actions on the regulation of intestinal secretion, depending on the subject’s nutritional status (fasted-fed). Orexin-A and Orexin-B peptides, are derived from the prepro-orexin precursor protein. These ligands bind to two G-protein-coupled receptors, orexin-1 and -2 -receptors. Despite intensive research, the role of orexins has not yet been clarified. The aim of the present study was to investigate the role of orexins and their receptors on sleep and wake patterns, energy homeostasis and intestinal secretion. The effects of orexins on sleep and wakefulness, and energy homeostasis were studied in a novel transgenic mouse line, overexpressing the human prepro-orexin gene. The overexpression of prepro-orexin and orexin-A was confirmed in the hypothalami of transgenic mice. The transgenic mice showed a significant reduction in their REM sleep during day and night time, and differences in their vigilance states in the light/dark transition periods. In addition, the mice demonstrated a significantly elevated day time food intake at room temperature, and an increased metabolic heat production independent of uncoupling protein 1 mediated thermogenesis in brown adipose tissue. Instead, transgenic mice showed increased levels of uncoupling protein 2 in white adipose tissue. Furthermore, transgenic mice significantly decreased their total locomotor activity during the first two nights in response to cold exposure (+4°C). The effect of orexins and their receptors on duodenal HCO3– secretion were studied after an overnight (16 h) food deprivation in an in situ perfused organ. Fasting decreased the expression of orexin receptors in rat duodenal mucosa and in acutely isolated duodenal enterocytes. Furthermore, food deprivation abolished OXA induced duodenal mucosal HCO3– secretion in rats, and intracellular calcium signalling in isolated rat and human duodenal enterocytes. In conclusion, the present thesis demonstrates that orexins inhibit REM sleep. In addition, peptides affect increasingly on metabolic heat production, independent of uncoupling protein 1 mediated thermogenesis. Furthermore, the orexin system has a significant role in duodenal bicarbonate secretion, which is regulated by the presence of food in the intestine.