Although studies of the principal tongue protrudor muscle, genioglossus (GG) suggest that whole muscle GG electromyographic (EMG) activities are preserved in NREM sleep, it is unclear what influence sleep exerts on individual GG motor unit (MU) activities. We characterized the firing patterns of human GG MUs in wakefulness and NREM sleep with the aim of determining a) whether the range of MU discharge patterns evident in wakefulness is preserved in sleep and b) what effect the removal of the wakefulness input has on the respiratory modulation of MU activities. Microelectrodes inserted into the extrinsic tongue protrudor muscle, the genioglossus (GG), were used to follow the discharge of single MUs. We categorized MU activities on the basis of the temporal relationship between the spike train and the respiration cycle and quantified the respiratory modulation of each MU using the eta index (Netick & Orem, 1981), in wakefulness and sleep. The majority of MUs exhibited subtle increases or decreases in respiratory modulation but were otherwise unaffected by NREM sleep. In contrast, 30% of MUs exhibited marked sleep-associated changes in discharge frequency and respiratory modulation. We suggest that GG MUs should not be considered exclusively tonic or phasic rather, discharge pattern appears to be a flexible feature of GG activities in healthy young adults. Whether flexibility is important in the response to changes in the chemical and/or mechanical environment and whether it is preserved in the aged or in individuals with OSA are critical questions for future research.