In addition, hyperleptinemia (elevated plasma leptin levels) and obstructive sleep apnea (OSA, characterized by recurrent upper-airway obstructions caused by a loss of pharyngeal muscle tone during sleep), two processes intimately connected with obesity, may also upregulate the production of ROS via the activation of NADPH oxidase or after recurrent cycles of hypoxemia and reoxygenation, respectively [30]. This evidence concerns the gene LEP and obstructive sleep apnea syndrome.