For example, (i) 4–6 days of chronic hypoxia results in significant decreases in UCP1 protein expression in neonatal and adult BAT in rats52,53, (ii) 4 weeks of chronic hypoxia increases UCP1 mRNA, but not UCP1 protein expression in mice54, and (iii) in a mouse model of sleep apnea, 37 days of daily 8 h periods of intermittent hypoxia decreases UCP1 mRNA expression and UCP1-positive cells in iBAT55. Here, UCP1 is linked to sleep apnea syndrome.