"Latency to REM sleep and percentages of light sleep and REM sleep were normal, compared with normal ranges." Despite this, one study has found increased sleep fragmentation in patients with IH without long sleep time, suggesting multiple possible presentations.Per ICSD-3, five criteria must be met for a diagnosis of IH:

Daytime lapses into sleep or an irrepressible need to sleep on a daily basis, for at least 3 months
Absence of insufficient sleep syndrome
Absence of cataplexy
Absence of other causes of hypersomnia
The presence of positive MSLT tests.The latest ICD 10 defines IH with long sleep time as a neurological disorder that is a rare sleep disorder characterized by prolonged sleep at night and extreme sleepiness during the day. There are no apparent causes. This disorder affects the ability to function. It is of central nervous system origin characterized by prolonged nocturnal sleep and periods of daytime drowsiness. Affected individuals experience difficulty with awakening in the morning and may have associated sleep drunkenness, automatic behaviors, and memory disturbances. This condition differs from narcolepsy in that daytime sleep periods are longer, there is no association with cataplexy, and the multiple sleep latency onset test does not record sleep-onset rapid eye movement sleep. Management
Since the underlying mechanism is not yet fully understood, treatment efforts have usually focused on symptom management. In August 2021, low-sodium oxybate (Xywav) became the first U.S. FDA-approved treatment for idiopathic hypersomnia. Wake-promoting medications used in narcolepsy are also commonly used off-label to help manage the excessive daytime sleepiness of IH.
One small study, which used a dose of 2 mg slow-release melatonin at bedtime, found that 50% of participants had “shortened nocturnal sleep duration, decreased sleep drunkenness and relieved daytime sleepiness.” Other studies have shown that melatonin synchronizes the circadian rhythms, and improves the “onset, duration and quality of sleep.”

Levothyroxine
There have been some studies suggesting levothyroxine as a possible treatment for IH, especially for patients with subclinical hypothyroidism. This treatment does carry potential risks (especially for patients without hypothyroidism or subclinical hypothroidism), which include cardiac arrhythmia. Hypocretin agonists
Hypocretin-1 has been shown to be strongly wake-promoting in animal models, but it does not cross the blood-brain barrier. Suvorexant, a hypocretin receptor antagonist, has been developed to limit the natural effects of hypocretin in patients with insomnia. It is therefore possible that a hypocretin agonist may be similarly developed for the treatment of hypersomnia. Acetylcholinesterase inhibitor
Antidepressants seems to have some therapeutic effects as they enhance synaptic levels of noradrenaline and serotonin and for same reason enhancing the levels of acetylcholine may have some therapeutic effect. Donepezil showed improvement in one patient by decreasing ESS score from 20 up to 14. Memantine has also shown some positive effect on a patient with narcolepsy. Levodopa
Levodopa is an amino acid and is the precursor to the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline).