Sluggish cognitive tempo (SCT) and daytime sleepiness are both common in individuals with attention-deficit/hyperactivity disorder (ADHD). There appears to be considerable overlap between the tired and lethargic aspects of SCT and behaviors frequently exhibited by individuals with daytime sleepiness. However, no studies have examined the degree to which these constructs overlap and whether or not they are empirically distinct. In Study 1, a confirmatory factor analysis with the SCT subscale of the Barkley Adult ADHD Rating Scale-IV (BAARS-IV) and the Epworth Sleepiness Scale (ESS) was conducted in a sample of 768 college students. Results demonstrated that SCT and daytime sleepiness exhibit considerable overlap but are empirically distinct. In Study 2, we examined the relation between SCT and daytime sleepiness and also the impact of comorbid SCT and sleepiness on the functioning of 58 college students rigorously diagnosed with ADHD. Regression analyses in both Study 1 and Study 2 showed that SCT predicts daytime sleepiness above and beyond symptoms of ADHD, anxiety, and depression. The 2 constructs were significantly related (r = .51), with the highest correlations occurring between the SCT tired and lethargic items with daytime sleepiness. College students with ADHD + SCT and daytime sleepiness were significantly more impaired than college students diagnosed with ADHD without SCT or daytime sleepiness. Together, these results fill an important gap in the literature by confirming SCT to be overlapping but empirically distinct from daytime sleepiness and demonstrating that SCT and daytime sleepiness are associated with functioning in college students with ADHD. (PsycINFO Database Record (c) 2014 APA, all rights reserved).