“…This interesting insight is based on their two different studies, in which a phase shift in circadian rhythm (as measured by cortisol and melatonin) was observed after eighthours-long 'chronic' exposure to hypoxia [7,8]. Coste & Touitou [6] carefully considered that hypoxia-based interventions could therefore induce changes in circadian rhythm, and this may in turn affect the outcome of these interventions. Here, we discuss important differences between chronic hypoxia, which resembles hypoxia as a disease model for sleep apnea, and hypoxic conditioning.…”