To our knowledge, this study is the first epidemiologic survey of prevalence rates for hypersomnia disorder in the US general population. Although reports of ES were common in our sample (27.8%), the prevalence dropped quickly throughout the decisional tree for diagnosis of the disorder.
Our study presents several unique observations. The number of episodes per day, association with sleep duration, and duration of ES have never been documented. Excessive sleepiness at least 3 times per week is common in the general population. Interestingly, multiple episodes of sleepiness within the same day were reported by half the individuals reporting ES. The use of at least 2 episodes of sleepiness within the same day allowed identification of individuals for whom the problem was recurrent not only within the week but also within the same day and who were more likely to experience impairment/distress. This pattern appears to be a stronger indicator of a hypersomnia disorder, especially when sleep duration is within a normal range. Our data also indicate that ES was frequently reported when nocturnal sleep duration fell below the normal length of 7 hours,15
suggesting the presence of another type of sleep disorder, such as insomnia or insufficient sleep.
Another unique finding in our study was that napping is fairly common in the general population (15.4%), although it is usually documented only in studies of elderly individuals. Reports of ES accompanied by naps were less frequent (7.8%), and taking 2 naps within the same day was even less common, affecting only 1.9% of the respondents. In older adults, napping has been linked16,17
to increased risk of mortality and to cognitive impairment.
Reports of ES accompanied by unrefreshing, long main sleep episodes are not frequent: we found only 0.7% of the general population with this symptom. Again, there is no comparison point with other epidemiologic studies. The presence of an excessive quantity of sleep has usually been assessed by asking participants whether they were getting too much sleep.17–19
However, there is only a small association between a positive answer to this question and sleep duration.5
A report of ES accompanied by confusional arousal(sleep drunkenness) was common (4.4%) in our sample. Confusional arousal is often associated with hypersomnia disorder; individuals with this disorder report difficulties in a wakening in the morning, disorientation, and confusion.20,21
The presence of one of the symptoms reported herein clearly is a requisite for the identification of a hypersomnia disorder. As many as 15.6% of the sample reported ES accompanied by at least 1 of these symptoms. This population-based description is more comprehensive than that in DSM-IV
and includes not only individuals with ES related to the quantity of sleep but also those with ES related to a decreased quality of wakefulness despite a normal sleep duration. Our measurement approach encompasses ICSD-21
diagnoses of idiopathic hypersomnia with long sleep time and idiopathic hypersomnia without long sleep time.
Another observation contributed by this study concerns the duration criterion: 3 months of ES is more clinically relevant than 1 month. Interestingly, the ICSD-2 also uses 3 months as a diagnostic criterion.
At the diagnostic level, this study used new criteria, encompassing an excessive quantity of sleep and quality of wakefulness to determine the prevalence of hypersomnia disorder in the general population. Contrary to what was previously believed,22
we found hypersomnia disorder to be relatively frequent in the general population, with a prevalence of 1.5% in our sample meeting this new definition. Two previous studies7,8
that used DSM-IV
classifications reported the prevalence of hypersomnia disorder to be approximately 0.3%. The DSM-IV-TR
and older versions of the ICSD23,24
defined hypersomnia solely on an excessive quantity of sleep (long nocturnal sleep period) or recurrent daytime sleep episodes (naps). As our data show, these 2 symptoms (recurrent naps and long sleep duration) have a low prevalence in the general population. Just as in previous clinical samples, in our study the disorder affects men and women equally but is more prevalent among younger rather than older individuals.
Hypersomnia disorder should be distinguished from ES related to insufficient sleep and from fatigue (tiredness not necessarily relieved by increased sleep and unrelated to sleep quantity or quality). Excessive sleepiness and fatigue are difficult to differentiate and may overlap considerably. Individuals with this disorder have no difficulty falling asleep and have a sleep efficiency generally higher than 90%. They may experience confusional arousal upon awakening in the morning but also upon awakening from a daytime nap. During that period, the affected individual appears to be awake, but behavior may be very inappropriate, with memory deficits, disorientation in time and space, and slow mentation and speech. This reduced vigilance and impaired cognitive response return to normal within 30 minutes to more than 1 hour. It is estimated25
that approximately 1% of the general population has episodes of confusional arousal. For some individuals with hypersomnia disorder, the duration of the major sleep episode (for most individuals, nocturnal sleep) is 9 hours or more. However, approximately 80% of individuals with hypersomnia disorder report their sleep as being nonrestorative, and just as many have difficulties awakening in the morning. Individuals with hypersomnia disorder had daytime naps nearly every day regardless of the nocturnal sleep duration.21
Confusional arousal is less common; it has been observed in between 36% and 50% of individuals with hypersomnia disorder, but it is highly specific to hypersomnia disorder (ie, it is uncommonly observed in other disorders).20,21,26
Short naps (<30 minutes) are often unrefreshing.21,26,27
In summary, at the basic level, reports of ES have a prevalence of approximately 27.8%. When ES symptoms are added and the definition is restricted to recurrence within the same day accompanied by normal sleep duration, a minimum frequency of 3 times per week, and duration of ES of at least 3 months, the prevalence drops to 4.7% and further to 1.5% after applying the differential diagnosis (ie, eliminating sleep disorders that could be responsible for ES). With the DSM-IV
definition, which used only excessive quantity of sleep, we would have a prevalence of 0.5% for hypersomnia disorder. This is close to the 0.3% found in European studies7,8
using the DSM-IV
definition of hypersomnia disorders. Our findings support our initial assumption that daytime wakefulness difficulties occurring with a normal sleep duration are a good indicator, increasing the prevalence of hypersomnia disorder.
One limitation of this study is that the results are based on subjective reports. Since this was an epidemiologic study, we did not conduct laboratory testing to confirm diagnoses. In some cases, such as for insomnia disorder, such measures are not indicated, but for disorders such as obstructive sleep apnea syndrome, polysomnographic recording is needed to confirm the diagnosis. Similarly, the use of the day time Multiple Sleep Latency Test28
accompanied by nocturnal polysomnographic recording is useful to confirm a diagnosis of narcolepsy without cataplexy. Hypocretin deficiency is measured using lumbar puncture, which obviously could not be measured in an epidemiologic study.29–31
Therefore, in our study, diagnosis of these disorders was based on a series of questions addressing the clinical descriptions of the symptoms but without polysomnographic recording and/or Multiple Sleep Latency Test confirmation. Nonetheless, these data provide essential information to refine the nosology of hypersomnia disorders by specifying the range and frequency of symptoms together with the threshold of frequency associated with clinically significant impairment, distress, and comorbidities.
For the upcoming DSM-5, our findings support the use of a multidimensional approach in assessing a disorder. To date, measures of the dimensional aspects of ES have been weak. Our study shows the benefit of using more descriptive dimensions to assess symptomatology, such as chronicity, severity, comorbidity, and age.
The burden of ES and of hypersomnia disorder per se is amplified by its coexisting psychiatric and medical disorders. Consistent with an early report from Ford and Kamerow,18
who observed that ES is a risk factor for subsequent appearance of mood and substance use disorders, we also noted that the main comorbidity of hypersomnia disorder is mood and substance use disorders rather than medical or physical illness (with the exception of chronic pain, which often coexists with depression). Thus, evaluation of the patient with ES needs to consider these important comorbid sources of disability in addition to sleep disorders, such as hypersomnia, breathing-related sleep disorder, insomnia disorder, and narcolepsy/hypocretin deficiency. Finally, the prevalence of ES in the general population is an alarming phenomenon that requires attention from public health authorities and clinicians.