Introduction: The role of a gluten-free diet (GFD) in Non-Coeliac Gluten/Wheat Sensitivity (NCGWS) is unclear. We present the largest study comparing adherence to a GFD in patients with Coeliac Disease (CD) and NCGWS and assess its impact on quality of life (QoL) and sleep in patients with NCGWS. Methods: Patients with NCGWS at a tertiary centre completed the Coeliac Disease Adherence Test (CDAT), Coeliac Symptom Index (CSI) and Sleep Condition Indicator (SCI). Higher CDAT scores indicate worse adherence, higher CSI scores indicate poorer QoL, and higher SCI scores indicate better sleep. CDAT scores were correlated with CSI and SCI scores. A second group of patients with CD completed the CDAT questionnaire only. Results were compared with the CDAT responses from the NCGWS group. Results: For the NCGWS cohort (n = 125), the median CDAT score was 17/35, indicating poor adherence. The median CSI score was 44/80, with 40% of scores associated with a poor QoL. The median SCI score was 14/32, and DSM-V criteria for insomnia was met by 42% of patients. There was a positive correlation between CSI and CDAT scores (r = 0.59, p < 0.0001) and a negative correlation between SCI and CDAT scores (r = −0.37, p = 0.0002). In the CD cohort (n = 170), the median CDAT score was 13/35. Patients with NCGWS had poorer adherence compared to CD (CDAT: 17.0 vs. 13.0, respectively, p = 0.0001). Conclusion: Patients with NCGWS adhere to a GFD less than those with CD. Poorer adherence to a GFD in patients with NCGWS correlates with a worse QoL and sleep performance.