Seasonality of excessive daytime sleepiness has been proposed, yet no research has specifically investigated its impact on daytime sleepiness and cataplexy in central disorders of hypersomnolence. This study examined seasonal variations in daytime sleepiness and cataplexy in narcolepsy type 1, narcolepsy type 2 and idiopathic hypersomnia. Patients included in the study were on stable pharmacological treatment, and participated in sleep medicine interviews to assess diurnal sleepiness and daytime napping and completed the Epworth Sleepiness Scale to assess excessive daytime sleepiness (Epworth Sleepiness Scale ≥ 10). Patients with narcolepsy type 1 also maintained a cataplexy diary. Evaluations were conducted in autumn, winter, spring and summer. The study included 29 patients with narcolepsy type 1, 16 patients with narcolepsy type 2 and 10 patients with idiopathic hypersomnia. Patients with narcolepsy type 1 and narcolepsy type 2 showed higher Epworth Sleepiness Scale scores in summer compared with other seasons, while patients with idiopathic hypersomnia showed no changes in excessive daytime sleepiness across the four seasons. Epworth Sleepiness Scale scores were higher in idiopathic hypersomnia patients compared to narcolepsy type 1 and narcolepsy type 2 patients in spring, autumn, and winter; conversely, in summer there were no differences in Epworth Sleepiness Scale scores among the three groups. No significant differences in Epworth Sleepiness Scale scores were noted between patients with narcolepsy type 1 and narcolepsy type 2 throughout the year. Furthermore, no seasonal effect on cataplexy frequency was found in patients with narcolepsy type 1. This study demonstrates that seasonality may influence daytime sleepiness in patients with narcolepsy type 1 and narcolepsy type 2 but not in patients with idiopathic hypersomnia, while cataplexy symptoms remain unaffected by seasonal changes. The underlying mechanisms linking excessive daytime sleepiness to seasonality have yet to be explored, though social factors and vacation time may contribute to increased excessive daytime sleepiness in narcolepsy.