Abstract Introduction Sleep and circadian rhythms are influenced by exposure to light at specific times of day. In particular, blue light exposure can suppress melatonin production, shift circadian timing of sleep and wake, and acutely enhance alertness. Despite the consistency of these effects, little is known about their underlying mechanisms. We propose that in addition to the melatonin and phase shifting effects of blue light, that it also produces acute changes in brain activation that lead to greater neural efficiency. Methods Twenty-six individuals (11 male; 15 female, Mean age=24.27, SD=6.27) completed a counterbalanced cross-over design study while undergoing two separate neuroimaging scans in a 3T MRI scanner separated by one week. During scanning, each participant was exposed to either BLUE light (470 nm; active condition) or AMBER (580 nm; placebo condition) light conditions on alternate weeks. All scans occurred between 11:00 a.m. and 1:30 p.m., a time that has been described as the “dead zone” when melatonin levels are generally unaffected by light exposure. Participants completed a well-established working memory task (i.e., N-back task) in the scanner while undergoing continuous exposure to the specific light wavelength for the duration of the task. We contrasted the simple 1-back memory condition versus the 0-back memory condition using SPM12. Contrast maps were then compared using a paired-samples t-test. Results Compared to AMBER light, the BLUE light was associated with significantly less deactivation within two large clusters comprising the default mode network (DMN). These included a large cluster (k=1343 voxels) in the medial prefrontal cortex and a large cluster (k=5075 voxels) encompassing the posterior cingulate, precuneus, and parietal cortex regions (p<.05 FDR cluster corrected). Melatonin levels did not differ from pre-to-post light exposure for either condition. Conclusion Despite no effect on salivary melatonin, BLUE light exposure was associated with significantly less deactivation of brain regions that are usually suppressed to engage in cognitively demanding tasks. This suggests that blue light appears to enhance cognitive efficiency, potentially leading to similar performance while taxing fewer brain resources. Such findings suggest a potential role for blue light in sustaining performance during periods of sleep loss. Support (If Any)