NEFL and Parkinson disease: In terms of daytime sleepiness, PD males who had a higher score of ESS (β = 0.002; P = 0.002), those who were considered as EDS (β = 0.017; P = 0.005) (Fig. 5K), and those had possibilities of daytime sleepiness on four conditions (sitting inactive in public places (β = 0.008; P = 0.010), as a passenger staying in a car for an hour without a break (β = 0.006; P = 0.015), lying down to rest in the afternoon (β = 0.005; P = 0.043), sitting and talking to someone (β = 0.017; P = 0.023)) also showed an increasing trend of in serum NfL levels.