Which disorder primarily causes excessive daytime sleepiness without REM sleep onset or cataplexy?

The condition described in the question is idiopathic hypersomnia, which is characterized by excessive daytime sleepiness that occurs without the occurrence of REM sleep onset or cataplexy. Individuals with idiopathic hypersomnia often have difficulty fully awakening after a sleep period, leading to prolonged sleep durations and significant daytime fatigue. Unlike narcolepsy, which features episodes of cataplexy and rapid onset REM sleep, idiopathic hypersomnia does not include these characteristics, making it distinct from narcolepsy.

Additionally, obstructive sleep apnea is characterized by disrupted breathing during sleep, which may lead to excessive daytime sleepiness but has different pathophysiological mechanisms, including frequent arousals and fragmented sleep architecture rather than the sustained sleepiness seen in idiopathic hypersomnia. Restless legs syndrome primarily affects the ability to stay asleep due to uncomfortable sensations in the legs and the irresistible urge to move, but it does not inherently cause excessive daytime sleepiness in the same way or with the same mechanisms as idiopathic hypersomnia. This illustrates why idiopathic hypersomnia is the correct answer to the question posed.