260 Chapter 9 phenotype, probably mediated by other (still largely unknown) pathophysiology or comorbid conditions. Redefining key symptoms may help to establish new diagnostic methods such as forced awakening, unrestricted/extended sleep opportunity, and other biomarkers. Sleep inertia is the complaint of difficulty in achieving complete wakefulness at the end of a sleep period, potentially accompanied by confusion, disorientation, and poor motor coordination or even ataxia. Sleep drunkenness is considered a severe manifestation of this phenomenon [25]. Historically speaking, sleep drunkenness had already been introduced by Roth [329] as an important differential marker when idiopathic hypersomnia was first conceptualized in the 1950s [26, 330]. To date, quality of awakening has consistently been undervalued by the ICSD criteria for central hypersomnolence disorders, despite multiple studies suggesting its importance in people without cataplexy [25, 26, 329-335]. Besides the presence of subjective sleep drunkenness by patient history, the sensitivity and specificity of standard vigilance testing directly before and after MSLT naps could be explored to quantify the level of sleep drunkenness. The Sustained Attention to Response Task [6, 93, 266] and Psychomotor Vigilance Test [13] have previously been used to quantify vigilance deficits in people with central hypersomnolence disorders but not yet in relation to sleep drunkenness. A smaller study including different central hypersomnolence disorders has also previously reported event-related potentials during forced awakening to potentially quantify sleep inertia [336]. Difficulties in waking up and increased sleep demand frequently, but not always, coexist in people without cataplexy [330, 331, 333, 334, 337]. Classically, these individuals were grouped in the ICSD-2 diagnosis idiopathic hypersomnia with long sleep time [7]. In our results, cluster 5 reported a substantially greater subjective weekend-week sleep length difference, a variable quantifying unfulfilled sleep need. People with increased sleep need are often unable to satisfy sleep requirements during the week because of professional/ social obligations, whereas the weekend generally allows opportunity for more unrestricted sleep [338]. The mean duration of nocturnal sleep during actigraphy may therefore not always reliably reflect the presence of an increased need for sleep. Moreover, in most sleep clinics, the MSLT routine prevents the objective confirmation of a long duration of nocturnal sleep because individuals are forced to wake early in the morning. We previously applied supervised machine learning to classify people with narcolepsy types 1 and 2 in the EU-NN database, and longer weekend-week sleep length difference was a more important deterministic parameter for narcolepsy type 2 than for type 1, further highlighting its potential to become a new clinical
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