Sleep Epidemiology Research & Sleep-EVALTM Diagnosis Expert System

Stanford Sleep Epidemiology Journal

Stanford Sleep Epidemiology Research Center (SSERC)

Psy-EVAL Research


"Not everything that can be counted counts,
and not everything that counts can be counted."


Last Findings

First created | 05/04/2009

Last edited   | 05/11/2012

Gates of Hell: Excessive Somnolence and Excessive Quantity of Sleep

Prevalence, Comorbidity and Mortality

A Stanford Sleep Epidemiology Center Symposium

April 27th 2012

At the Stanford Sleep Epidemiology Research Center (SSERC)

3430 West Bayshore Road


Why a meeting on Excessive Somnolence (ES) and Excessive Quantity of Sleep (EQS)? Hypersomnolence (DSM-5) is a broad diagnostic term and includes symptoms of Excessive Quantity of Sleep (e.g., extended nocturnal sleep or involuntary daytime sleep) and Excessive Somnolence (e.g. deteriorated quality of wakefulness as a sleep propensity during wakefulness and an inability to remain awake when required). The change from Sleepiness to Somnolence is not only cosmetic for one of the main problems encountered in the study of Excessive Daytime Sleepiness is the lack of uniformity of its definition. Somnolence is more accurate in the fact that its main expression is easily understandable and directly linked with the Hypersomnolence Disorder as recently defined by the DSM-5.

While there is a mounting evidence of the high prevalence of Excessive Somnolence in the community, its meaning in term of medical/psychological consequences remains understudied.

There is a growing trend in labeling Excessive Somnolence as a disease or a disorder. So far, there is no data supporting this claim. Excessive Somnolence is not a disease or a disorder: it is a symptom of a sleep disorder or of another disease and for sure part of Hypersomnolence Disorder. There is some scientific evidence linking Excessive Somnolence to cognitive deficits and Excessive Quantity of Sleep to increased mortality risk in the community, but the links with specific sleep disorders remain to be established.

During this meeting, the participants will discuss on several aspects related to comorbidity, mortality and the mechanisms involved in Hypersomnolence Disorders. At the end of this meeting we will have a clearer picture on how we should orient our research efforts to improve our understanding of Hypersomnolence, how it should be measured and what we still have to learn in order to improve the quality of life of people complaining of Excessive Somnolence or Excessive Quantity of Sleep.

More Information: Stanford Sleep Epidemiology Journal


Epidemiology of restless legs syndrome: A synthesis of the literature


Restless legs syndrome (RLS) has gained considerable attention in the recent years: nearly 50 community-based studies have been published in the last decade around the world.

The development of strict diagnostic criteria in 1995 and their revision in 2003 helped to stimulate research interest on this syndrome.


In community-based surveys, RLS has been studied as:

1) a symptom only,

2) a set of symptoms meeting minimal diagnostic criteria of the international restless legs syndrome study group (IRLSSG),

3) meeting minimal criteria accompanied with a specific frequency and/or severity, and

4) a differential diagnosis.


- In the first case, prevalence estimates in the general adult population ranged from 9.4% to 15%.

- In the second case, prevalence ranged from 3.9% to 14.3%.

 - When frequency/severity is added, prevalence ranged from 2.2% to 7.9% and when differential diagnosis is applied prevalence estimates are between 1.9% and 4.6%.

- In all instances, RLS prevalence is higher in women than in men.


It also increases with age in European and North American countries but not in Asian countries.

Symptoms of anxiety and depression have been consistently associated with RLS.

Overall, individuals with RLS have a poorer health than non-RLS but evidence for specific disease associations is mixed.


Future epidemiological studies should focus on systematically adding frequency and severity in the definition of the syndrome in order to minimize the inclusion of cases mimicking RLS.

Ohayon MM., O'Hara R., Vitiello MV. Epidemiology of restless legs syndrome: A synthesis of the literature. Sleep Med Rev. 2011 Jul 25. [Epub ahead of print]

Demographic factors, fatigue, and driving accidents


Fatigue has been implicated in a range of impairments that can have detrimental effects on individuals, and it is differentially associated with conventional demographic variables. However, several major methodological limitations prevent clear conclusions: - First, there is absence of a shared definition both within and across disciplines. - Second, although fatigue has been investigated using a variety of diverse designs, they have either been too weak to substantiate causality or lacked ecological validity. - Third, while both subjective and objective measures have been used as dependent variables, fatigue has been more often found to be more strongly linked with the former. - Fourth, with the exception of age and sex, the influence of other demographic variables is unknown, since they have not yet been concomitantly assessed. In instances when they have been assessed and included in statistical analyses, they are considered as covariates or confounders; thus, their contribution to the outcome variable is controlled for, rather than being a planned aspect of investigation. Because the interaction of demographic factors with fatigue is largely a neglected area of study, it is recommended greater interdisciplinary collaborations, incorporation of multiple demographic variables as independent factors, and use of within-participant analyses. These recommendations would provide meaningful results that may be used to inform public policy and preventive strategies.

Di Milia L, Smolensky MH, Costa G, Howarth HD, Ohayon MM, Philip P. Demographic factors, fatigue, and driving accidents: An examination of the published literature. Accid Anal Prev. 2011 Mar;43(2):516-32.



Sleep disorders, medical conditions, and road accident risk


Although numerous investigations have addressed the impact of sleep and medical disorders on quality of life, few have specifically addressed their potential deleterious effect on driving performance and road incidents. Moreover, since past studies have focused on the survivors of driver crashes, they may be biased.

Representative population-based prospective multidisciplinary studies are urgently required to clarify the role of the fatigue associated with common ailments and medications on traffic crash risk of both commercial and non-commercial drivers and to comprehensively assess the cost-effectiveness of intervention strategies.

Smolensky MH, Di Milia L, Ohayon MM, Philip P. Sleep disorders, medical conditions, and road accident risk. Accid Anal Prev. 2011 Mar;43(2):533-48.



Nocturnal Awakenings and Difficulty Returning to Sleep (DRS) have heavy daytime consequences

Individuals with painful physical condition or with a psychiatric disorder were more than four times more likely to have DRS.
Nocturnal awakenings are highly prevalent in the general population and strongly associated with various physical diseases and psychiatric disorders.
There is also a dose response effect in the associations: odds ratios increased with the number of awakenings during the same night and the difficulty resuming sleep once awakened.
The study shows that nocturnal awakenings are complex and should be assessed systematically.

Ohayon MM. Nocturnal awakenings and difficulty resuming sleep: their burden in the European general population. J Psychosom Res. 2010 Dec;69(6):565-71.


Prevalence of insomnia and sleep characteristics in the general population of Spain

Participation rate was 87.5%.

Overall, 20.8% (95% C.I. 19.6-22.1%) of the sample reported at least one insomnia symptom occurring at least three nights/week.

The prevalence was higher in women than in men (23.9% vs. 17.6%) and increased with age.

Difficulty maintaining sleep at least three nights/week was the most prevalent symptom.

DSM-IV insomnia disorder diagnoses were found in 6.4% (95% C.I. 5.6-7.1%) of the sample.

Delayed bedtime and wake-up time, irregular bedtime hours and hypnagogic hallucinations were the most frequent in participants who had difficulty initiating sleep.

Perception of light and "too short" sleep were the most frequent in participants who had early morning awakenings.

Participants who had a non-restorative sleep were more likely to extend sleep on days off than other insomnia participants. Medical consultations in the previous year were more frequent in insomnia participants compared to participants without insomnia.

One-fifth of insomnia participants were using sleep-promoting medication.

Ohayon MM, Sagales T. Prevalence of insomnia and sleep characteristics in the general population of Spain. Sleep Med. 2010 Dec;11(10):1010-8. Epub 2010 Nov 18.


Violent behaviors during sleep are frequent in the general population and often associated with dream-enactment, sleepwalking and sleep terrors

Random stratified sample of 19,961 participants

VBS was reported by 1.6% (95% confidence interval: 1.4-1.7%) of the sample.

VBS was higher in subjects younger than 35 years.

During VBS episodes, 78.7% of VBS subjects reported vivid dreams and 31.4% hurt themselves or someone else.

Only 12.3% of them consulted a physician for these behaviors.

In 72.8% of cases, VBS were associated with other parasomnias (highest odds of VBS for sleepwalking and sleep terrors).

Family history of VBS, sleepwalking and sleep terrors was reported more frequently in VBS than in non-VBS subjects with odds of 9.3, 2.0 and 4.2, respectively.

VBS are frequent in the general population and often associated with dream-enactment, sleepwalking and sleep terrors.

High frequency of VBS, sleepwalking and sleep terrors in family of VBS subjects indicated that some families have a greater vulnerability to sleep disorders involving motor dyscontrol.

Subjects who consulted a physician for these behaviors mostly received inappropriate or no support, indicating a lack of knowledge about VBS.

Ohayon MM, Schenck CH. Violent behavior during sleep: prevalence, comorbidity and consequences. Sleep Med. 2010 Oct;11(9):941-6.

Prevalence and correlates of insomnia in the Swedish population

Participation rate was 71.3%.

32.1% (95% confidence interval: 29.5-34.8%) of the sample reported having difficulty initiating (DIS) or maintaining sleep (DMS) or non-restorative sleep accompanied with sufficient sleep (NRS) at least 4 nights per week: 6.3% of the sample had DIS, 14.5% had DMS and 18.0% had NRS.

Results from logistic regressions showed that restless legs symptoms, breathing pauses during sleep and depressive or anxious mood were associated with DIS and DMS but not NRS.

Living in an urban area (OR:2.0) and drinking alcohol daily (OR:4.6) were associated only with NRS. Daytime symptoms were reported by over 75% of subjects with insomnia symptoms.

DIS, DMS and NRS were associated with daytime fatigue but not excessive sleepiness as measured by the Epworth scale.

DIS was associated with the use of sleeping pills or natural sleeping aid compounds in multivariate models.

Insomnia symptoms occurring at least 4 nights per week are frequent in Sweden, affecting about a third of the population. Subjects with NRS have a distinctly different profile than those with DIS or DMS, which suggests different etiological causes for this symptom.

Ohayon MM, Bader G. Prevalence and correlates of insomnia in the Swedish population aged 19-75 years. Sleep Med. 2010 Dec;11(10):980-6. Epub 2010 Nov 18.


Sleep epidemiology 30 years later: where are we?


Ohayon MM, Guilleminault C, Chokroverty S. Sleep epidemiology 30 years later: where are we? Sleep Med. 2010 Dec;11(10):961-2. Epub 2010 Nov 16.


Working outside the regular daytime hours is strongly associated with shorter sleep duration, sleepiness, and driving accident risk

Rotating shift and permanent night work arrangements are compromising sleep.
A representative sample of the general population of the state of New York--3,345 individuals > or = 18 yrs of age--was interviewed by telephone regarding their sleep and psychiatric and organic disorders.
Multivariate models were applied to derive odds ratios (OR) after adjustment for age, sex, physical illness, mental disorders, obstructive sleep apnea, and sleep duration.
On average (+/-SE), workers slept 6.7 +/- 1.5 h, but 40% slept < 6.5 h/main sleep episode.
Short-sleep duration (< 6 h) was strongly associated with fixed night (OR: 1.7) and day-evening-night shiftwork arrangement (OR: 1.9).
Some 20% of the workers manifested excessive sleepiness in situations requiring high attention, and it was associated with the fixed night (OR: 3.3) and day-evening-night work arrangements (OR: 1.5).
Overall, 5% of the workers reported sleep attacks; however, they occurred three-times more frequently in the fixed night (15.3%) than other work arrangements (OR: 3.2).
Driving accidents during the previous 12 months were reported by 3.6% of the workers and were associated with fixed night (OR: 3.9) and day-evening-night (OR: 2.1) work schedules.
The findings of this study indicate that working outside the regular daytime hours was strongly associated with shorter sleep duration, sleepiness, and driving accident risk. Night work is the most disrupting, as it is associated with insufficient sleep during the designated rest span and excessive sleepiness and sleep attacks during the span of activity, with an associated consequence being increased driving accident risk.
Ohayon MM, Smolensky MH, Roth T. Consequences of shiftworking on sleep duration, sleepiness, and sleep attacks. Chronobiol Int. 2010 May;27(3):575-89.

Sleep disorders and accidental risk in a large group of regular registered highway drivers

Despite convincing evidence regarding the risk of highway accidents due to sleepiness at the wheel, highway drivers still drive while sleepy.

Out of 37,648 questionnaires completed by frequent highway users (registered in an electronic payment system), we ran our analyses on 35,004 drivers who responded to all items.

Of all drivers, 16.9% complained of at least one sleep disorder, 5.2% reported obstructive sleep apnea syndrome, 9.3% insomnia, and 0.1% narcolepsy and hypersomnia; 8.9% of drivers reported experiencing at least once each month an episode of sleepiness at the wheel so severe they had to stop driving.

One-third of the drivers (31.1%) reported near-miss accidents (50% being sleep-related), 2520 drivers (7.2%) reported a driving accident in the past year, and 146 (5.8%) of these driving accidents were sleep-related.

The highest risk of accidents concerned patients suffering from narcolepsy and hypersomnia (odds ratio 3.16, p<.01) or multiple sleep disorders (odds ratio 1.46, p<.001).

Other major risk factors were age [18-30 years (OR 1.42, p<.001)] and being unmarried (OR 1.21-fold, p<.01).

In regular highway drivers, sleepiness at the wheel or sleep disorders such as hypersomnia and narcolepsy are responsible for traffic accidents.

Philip P, Sagaspe P, Lagarde E, Leger D, Ohayon MM, Bioulac B, Boussuge J, Taillard J. Sleep disorders and accidental risk in a large group of regular registered highway drivers. Sleep Med. 2010 Dec;11(10):973-9.

Difficulty Resuming Sleep (DRS) is predictive of significant daytime impairment

Cross-sectional telephone study with a representative sample consisting of 8937 non-institutionalized individuals aged 18 or over living in Texas, New York and California.

A total of 35.5% of the sample reported awakening at least three nights per week.

From this 35.5%, 43% (15.2% of the total sample) reported difficulty resuming sleep once awakened.

More than 80% of subjects with insomnia symptoms (difficulty initiating or maintaining sleep or non-restorative sleep) also had nocturnal awakenings.

Difficulty resuming sleep was associated with subjective shorter sleep duration, poorer sleep quality, greater daytime impairment, greater consultations for sleep disturbances and greater likelihood of receiving a sleep medication.

Nocturnal awakenings disrupt the sleep of about one-third of the general population.

Using difficulty resuming sleep identifies individuals with significant daytime impairment who are most likely to seek medical help for their sleep disturbances. In the absence of other insomnia symptoms, nocturnal awakenings alone are unlikely to be associated with daytime impairments.

Ohayon MM, Krystal A, Roehrs TA, Roth T, Vitiello MV. Using difficulty resuming sleep to define nocturnal awakenings. Sleep Med. 2010 Mar;11(3):236-41.


Social phobia increases the risk of developing a depression

18,980 individuals, aged 15 years or older, representative of the general population of the United Kingdom, Germany, Italy, Spain and Portugal.

The point prevalence for social phobia was 4.4% (95% confidence interval: 4.1-4.7%) of the sample. It was higher in women (odds ratio: 1.6) and decreased with age.

MDDs were found in 19.5% of participants with social phobia.

Co-occurrence of another anxiety disorder was high and increased when a MDD was present (65.2%).

The odds of developing a major depressive episode 2 years after the appearance of the social phobia was of 5.74.

Social phobia is highly prevalent in the general population.

It increases the risk of developing a MDD and has a high comorbidity with other mental disorders. Social phobia is often present in the course of depression, more obviously during remission period of MDD. Physicians must explore and treat more systematically this frequent pathology.

Ohayon MM, Schatzberg AF. Social phobia and depression: prevalence and comorbidity. J Psychosom Res. 2010 Mar;68(3):235-43.

Pain is predictive of depression

3243 subjects (from 18 years old), representative of California inhabitants, were interviewed by telephone. CPPC duration was at least 6months.
The point prevalence of CPPC was 49% (95% confidence interval: 47.0-51.0%).
Back area pain was the most frequent; 1-month prevalence of MDD was at 6.3% (95% CI: 5.5-7.2%); 66.3% of MDD subjects reported at least one CPPC.
In 57.1% of cases, pain appeared before MDD.
Pain severity was increased by poor sleep, stress and tiredness in MDD subjects. Being confined to bed, taking sick leave and interference of pain with daily functioning were twice as frequent among MDD subjects with CPPC than in non-MDD subjects with CPPC; obese individuals with CP were 2.6 times as likely to have MDD.
Pain is highly linked with depressive disorder. It deteriorates physical, occupational and socio-professional activities.
Pain and sleep disturbances are a prime motive of consultation rather than depressed mood, underlining the risk of missing a depression diagnosis.
Ohayon MM, Schatzberg AF. Chronic pain and major depressive disorder in the general population. Psychiatr Res. 2010 May;44(7):454-61.



Existing cross-sectional epidemiological studies tend to examine associations between insomnia and mental disorders only in a bi-dimensional way.
However, many other factors related to both insomnia and mental disorders might have confounding effects on this association. One of the possible consequences would be an amplification effect on odds ratios when no control is exerted on possible confounding variables. Two Venn’s diagrams are presented and show that insomnia symptoms and mental disorders are also often associated with several organic diseases and chronic pain.

A longitudinal study of a representative sample of the Californian general population was undertaken in 2005 and 2006 in our Center. The baseline sample included 3,249 adults from the general population interviewed between 2002 and 2003. At the end of the interview, they were asked if they were willing to be interviewed again in the future; 2,729 agreed to be part of the longitudinal survey.

The follow-up was done 3 years after the initial interview. A total of 1,957 individuals of the baseline sample was reached. Among participants lost to follow-up: 17 had died; 41 were seriously ill or hospitalized; 379 numbers were disconnected; 132 refused to participate and 203 numbers were attributed to a new household.

In this longitudinal study, we found that insomnia, psychiatric disorders, organic diseases and pain are closely interrelated.
Major depressive disorder at baseline was the strongest predictor of developing insomnia.
Insomnia at baseline was associated with 3 times the risk of developing a major depressive disorder and anxiety at 3-year follow-up.

Ohayon MM Observation of the Natural Evolution of Insomnia in the American General Population Cohort. Sleep Med Clin. 2009 March; 4(1) 87–92.

More Information


Study on Spectrum of Narcolepsy

We are inviting you to participate in a research study involving 4,000 children and adults. If you have narcolepsy or someone in your family has narcolepsy, we are asking you to participate in this study.

Research Aims
The Sleep-EVAL Research is a scientific program which was launched in 1990

Sleep Epidemiology
Various epidemiological studies have been conducted in the last fifteen years in the general population, still, many domains in the field of sleep epidemiology remain to be explored, in particular the repercussions of poor sleep on society

Questioning your sleep
Is it normal to sleep so much, so little, to snore, to scream, to walk, to kick, to punch during sleep?

Public Health Issues
It is estimated that sleep disorders, sleep deprivation, and sleepiness added $16 billion to the U.S. national health care bill in the early 1990’s