Daytime functioning is negatively affected with sleep disorders and reliable scales could help in screening and diagnosing, and would help as well in providing help as early as possible.
I would suggest that you consider going beyond fatigue as a measure of daytime impact of sleep disorders, and also include general measures of quality of life (QOL). In my own work, I have found that QOL is a sensitive indicator of treatment effects for insomnia
We've used the Multidimensional Assessment of Fatigue (MAF) and FOSQ in the past, and they seem to be good indicators of fatigue in terms of daytime functioning.