Introduction

Epidemiological studies in industrialized countries have reported prevalence rates of insomnia ranging between 10% and 48% [29]. Reported rates may vary because of differences in ascertainment procedure or in criteria definition. Studies employing quantifiers of insomnia symptoms such as frequency, severity, or consequences produce higher rates than those in which a less restrictive definition is used [29]. A study conducted in Brazil found rates varying from 12.0% to 76.3% for different definitions of insomnia [34]. An earlier study in France reported a rate of 18.6% for insomnia complaints, 12.7% for insomnia complaints lasting at least one month and accompanied by daytime repercussions, and 5.6% when DSM-IV criteria were used [28]. However, it is possible that differences in rates may be due to other factors such as culture. For example, insomnia symptoms were reported by more than a third of participants in a Finnish study, an occurrence rate that was about one and a half to two times higher than among subjects in four other western European countries in which similar methodology was used [30]. Difficulty maintaining sleep is the most commonly reported symptom. The Finnish study reported a rate of 11.9% for difficulty initiating sleep, 11.0% for early morning awakening, 7.9% for non-restorative sleep compared to 31.6% for difficulty maintaining sleep [30]. In the National Comorbidity Survey Replication, conducted in the US, the prevalence of difficulty initiating sleep was 16.4%, that of difficulty maintaining sleep was 19.9%, while early morning awakening had a prevalence of 16.7% [35].

Insomnia is often associated with social, demographic, and health variables [29]. Prevalence rate has consistently been reported to be higher among females [21, 31], with a women/men ratio of about 1.4 being the norm [29]. The prevalence of insomnia is also commonly reported to rise with age [1, 3, 22, 25], reaching close to 50% in persons aged 65 years and over [29]. Among social factors often linked with insomnia are marital separation or divorce, unemployment [2, 6, 27], low income [3], and low education [27].

Both physical and mental disorders are common among persons with insomnia. Multiple health problems are indeed not uncommon [22]. Among physical problems that have been reported are chronic pain [2], arthritis [27], heart disease [8], as well as obstructive airway diseases [20]. However, even stronger relationships have been described between insomnia and mental disorders, particularly depressive disorder. Insomnia is found in up to 80% of persons with current major depression [29] and insomnia complaints substantially increase the risk of developing major depressive illness [8].

Insomnia has also been associated with increased health care utilization, impaired quality of life [9, 14, 41] and poor self-rated overall health [18]. However, these associations may be confounded by the relationship between insomnia and factors such as age, physical and mental disorders which may themselves be risk factors for increased health care utilization, impaired quality of life, and poorer perception of own health.

In spite of the public health significance of insomnia, little is known about its epidemiology in Africa. Indeed, most previous studies have been conducted in industrialized western countries [29, 34]. Specifically, no studies are known to us that have addressed the frequency, pattern and functional correlates of insomnia in a large and representative community sample of Africans. Also, even though acknowledged as a cause of significant impairment in quality of life and of considerable economic cost [39], the specific decrement in functional limitation that may be due to insomnia has not been systematically explored. Such exploration is important in efforts to quantify the disability component of the burden that insomnia constitutes to societies.

In this report, we present the results of a general population study of insomnia complaints in a regionally representative sample of Nigerians. We report on the prevalence of insomnia complaints and its sociodemographic correlates. Using role limitation in the prior month as an index of disability, we evaluate the level of decrement in daily functioning that may be directly attributable to insomnia.

Method

Sample

The Nigerian Survey of Mental Health and Wellbeing (NSMHW) is a community based survey of the prevalence, impact, and antecedents of mental disorders that was conducted between February 2002 and May 2003 [10, 38]. It used a four-stage area probability sampling of households to select respondents aged 18 years and over.

The survey was conducted in five of the six geo-political regions of Nigeria: south-west (Lagos, Ogun, Osun, Oyo, Ondo, and Ekiti), south-east (Abia, Anambra, Enugu, Ebonyi, and Imo), south-south (Akwa Ibom, Cross-River and Rivers), north-central (Kaduna, Kogi, and Kwara), and north-east (Adamawa, Bornu, Gombe, and Yobe). Collectively, these states represent about 57% of the national population. In the first stage of the sampling, selection was made from an ordered list of all Primary Sampling Units (PSUs) stratified on the basis of states and size. Each PSU was a Local Government Area, a geographic unit with a defined administrative and political structure. In the second stage, four Enumeration Areas (EAs) were systematically selected from each PSU. EAs are geographic units of LGAs and consist of between 50 and 70 housing units. They are a creation of the National Population Commission and are used by the Commission in the conduct of national census.

All selected EAs were visited by research interviewers prior to the interview phase of the survey and an enumeration and listing of all the household units contained therein was conducted. These lists were entered into a centralized computer data file, thus creating sample in which the probability of any individual household being selected to participate in the survey was equal for every household within an EA. In the final stage of the selection, which was conducted during the interview phase of the survey, interviewers obtained a full listing of all residents in the household from an informant. After identifying household residents who were aged 18 years or over and were fluent in the language of the study, (Yoruba, Hausa, Igbo, or Efik, depending on the part of the country), a probability procedure was used to select one respondent to be interviewed. The Kish table selection method was used to select one eligible person as the respondent [24]. Only one such person was selected per household, except for a random 25% of households in which a secondary respondent, a spouse of the primary respondent who had been interviewed, was also selected for a study of assortative mating. When the primary respondent was either unavailable following repeated calls (5 repeated calls were made) or refused to participate, no replacement was made within the household.

On the basis of the selection procedure, face-to-face interviews were carried out on 6,752 respondents. The overall response rate was 79.3%. The survey was administered in two parts. Part I consisted of a core of diagnoses and was administered to all respondents. Part II consisted of sections for the assessment of risk factors, consequences and correlates of disorders as well as a few non-core disorders such as sleep. Part II was administered to respondents who met lifetime Part I disorders plus a probability sub sample of other respondents. Part II was administered to respondents who met life time part I disorders plus probability sub-sample of other respondents. The resulting Part II sample was composed of 2,143 respondents. Table 1 shows the age and sex distribution of the sample, weighted and unweighted. The weighted proportions approximate very closely to the United Nations 2000 projections of the Nigerian population (based on the last national census held in 1991).

Table 1 Demographic distribution of the sample compared to the population on post-stratification variables
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Respondents were informed about the study and provided consent, mostly verbal but sometimes signed, before interviews were conducted. The survey was approved by the University of Ibadan/University College Hospital, Ibadan Joint Ethical Review Board.

Measures

Diagnostic assessment was made with the use of the World Health organization’s (WHO) World Mental Health (WMH) Survey Initiative version of the Composite International Diagnostic Interview (CIDI) [17]. The CIDI is a fully structured diagnostic interview that is lay-administered and can generate diagnoses according to both the ICD-10 and DSM-IV criteria. The Yoruba version of the WMH-CIDI used in the present survey was derived, as in earlier versions [12, 13], using standard protocols of iterative back translation conducted by panels of bilingual experts. The WMH-CIDI primarily ascertains lifetime disorders. For respondents with lifetime occurrence of a disorder, follow-up questions allow a determination of whether they have also experienced such disorders in the prior 12 months. Other than insomnia problems, the questionnaire also identifies anxiety disorders (panic disorder, generalized anxiety disorder, agoraphobia without panic disorder, specific phobia, social phobia, post-traumatic disorder, obsessive-compulsive disorder), mood disorders (major depressive disorder, dysthymia, bipolar disorder), and substance use disorders (alcohol and drug abuse and dependence). In this report, DSM-IV organic exclusion rules were applied to the diagnoses of anxiety, mood and substance use disorders and so were hierarchy rules except in the case of substance use disorders where abuse is defined with or without dependence.

For the assessment of insomnia, the WMH-CIDI [27] asks the following questions:

“Did you have a period lasting two weeks or longer in the past 12 months when you had any of the following problems with your sleep?

  1. (1)

    Problems getting to sleep, when nearly every night it took you two hours or longer before you could fall asleep?

  2. (2)

    Problems staying asleep, when you woke up nearly every night and took an hour or more to get back to sleep?

  3. (3)

    Problems waking too early, when you woke up nearly every morning at least two hours earlier than you wanted to?

  4. (5)

    About how many weeks in the past 52 did you have problems like these with your sleep?”

Responses were coded as “yes” or “no”, except for the last question to which duration in weeks was obtained.

Insomnia was defined as the presence of any of these symptoms in the previous 12 months.

In this report, we present the associations of insomnia complaints with age, sex, education, per capita income, and residence. Per capita income was calculated by dividing household income by the number of people in the household. Respondents’ per capita income has been categorized by relating each respondent’s income to the median per capita income of the entire sample. Thus, an income is rated low if its ratio to the median is 0.5 or less, low-average if the ratio is 0.5–1.0, high average if it is 1.0–2.0, and high if it is over 2.0.

Disability

We employed a measure of role disability derived from the World Health Organization Disability Assessment Schedule [33], which is embedded in the WMH-CIDI [6]. The item asks the following questions:

How many days out of the last 30...

  1. 1

    ...were you totally unable to work or carry out normal activities?

  2. 2

    ...were you able to work or carry out normal activities, but had to cut down on what you did, or not get as much done as usual?

  3. 3

    ...did you cut back on the quality of your work or how carefully you worked?

  4. 4

    ...did it take extreme effort to perform up to your usual level at work or at your other normal daily activities?

Respondents who reported no disability days on these four items received a score of 100% for role performance. We calculated reduction in role performance by subtracting from 30 the reported disability days. Thus, a full day was subtracted for each day of total inability to work. A half day was subtracted for each day a person cut down on activities. A quarter day was subtracted for each day a person cut back on the quality of work, and a quarter day for each day a person took extreme effort to perform at the usual level. We set a maximum of 30 days for possible substraction from 30. The resulting number of role performance days was then divided by 3 and multiplied by 10, such that the resulting role performance score ranged from 0 to 100, representing the estimate of the percent of full role performance.

Data analysis

In order to take account of the stratified multistage sampling procedure and the associated clustering, weights have been derived and applied to the prevalences presented in this report. The analysis has taken account of the complex sample design and weighting. Simple cross-tabulations were used to calculate proportions and their distributions in different groups. To take account of the sampling procedure, with clustering and weighting of cases, standard errors of proportions were estimated with the jackknife method implemented in the STATA software [36]. Demographic correlates were explored with logistic regression analysis [15] and the estimates of standard errors of the Odds Ratio (ORs) obtained were made with the STATA All of the confidence intervals reported are adjusted for design effects.

We examined the association of insomnia with role limitation with linear regression [23]. We first computed the mean difference in role performance between persons with insomnia and those without insomnia. We next derived mean differences after adjusting for sociodemographic variables, chronic pain conditions, self-reported chronic physical conditions, and DSM-IV mental disorder, first singly and then combined.

Results

Prevalence of insomnia complaints

The prevalence of insomnia complaints ranged from 5.4% for early morning awakening to 8.5% for difficulty maintaining sleep (Table 2). Any insomnia complaint in the previous 12 months was reported by 11.4%. Increasing age was associated with every type of insomnia complaint. Significant gender difference was noted only in regard to difficulty initiating sleep, with higher prevalence found among females. The mean duration of any insomnia in the previous 12 months was 6.75 (s.d. 11.18) weeks.

Table 2 Prevalence of insomnia by demographic variables
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Risk factors

Other than the elevated risk of insomnia complaints with increasing age, sociodemographic features were generally not consistently associated with insomnia (Table 3). A trend for insomnia to be less commonly reported by persons in the low average income group, compared with those in the high income group, only reached statistical significance for early morning awakening. Compared with persons with 13 years or more of formal education, those with 1–6 years education were less likely to report difficulty-maintaining sleep. Not surprisingly, the presence of chronic pain, self-reported chronic medical disorder (hypertension, diabetes, chronic respiratory disorder, or any heart disease), and of any DSM-IV anxiety, mood or substance use disorders in the previous 12 months were independent risk factors for every type of insomnia. The presence of chronic pain bore the most consistently high relationship with every insomnia complaint, with persons with chronic pain conditions being three times more likely to report insomnia than those without chronic pain.

Table 3 Risk factors for insomnria
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Insomnia and role impairment

Table 4 displays the results of analyses exploring the relationship of insomnia to functional role impairment. Compared to persons with no insomnia, those with any insomnia had about 7.5% decrement in role performance in the prior month. The level of decrement was slightly reduced when adjustments were made for age, but the reduction was still statistically significant. The same pattern was seen when adjustment was made for chronic pain, presence of any DSM-IV mental disorder, or for self-reported chronic medical condition. When all of these demographic and health factors were controlled for, a 5.8% decrement in role performance was associated with the report of any insomnia of at least 2 weeks duration in the previous 12 months.

Table 4 Percent of full role performance for persons with and without any insomnia and reduction in role performance associated with insomnia
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Discussion

In this report, we have presented the results of a large community study of insomnia which, to our knowledge, is the first such study in sub-Saharan Africa in which ascertainment procedures comparable to those of other community-based surveys in Europe and North America have been used. Our methodology thus affords us the opportunity to compare our results with those of previous reports.

We obtained a rate of 11.8% for any insomnia complaint lasting at least two weeks in the previous 12 months. On average, persons with such complaints have had them for almost 7 weeks. Our rate is substantially lower than those of between 29.0% and 45.2% suggested by some previous reports in which insomnia had been defined without reference to duration or to frequency of symptom [3, 32, 41]. Conversely, studies employing stricter criteria such as frequency of symptom of at least three times a week reported lower prevalence [9, 27]. Studies employing DSM-IV criteria, which include a one-month duration, have obtained rates not very dissimilar to those reported here [4, 30]. We have employed a relatively strict criteria for our definition of any insomnia requiring the presence of at least one symptom occurring nearly everyday two weeks or longer but not DSM-IV criteria in view of our specific interest to evaluate the association of insomnia with role impairment. Embedded in DSM-IV criteria is a requirement for disablement related to the experience of insomnia. In this regard, our definition is closer to that described by Ohayon [29] as that based on insomnia symptoms only except that here, a minimum duration of two weeks of occurrence in the previous 12 months was included.

A more comparable report is the one presented by Kessler et al. [35] in which similar ascertainment procedure was used and same definitions were employed. Those authors reported a rate of 16.4% for difficulty initiating sleep, 19.9% for difficulty maintaining sleep, and 16.7% for early morning awakening. Our respective rates of 7.7%, 8.5%, and 5.4% are substantially lower than theirs. It would therefore appear that the lower rates we have obtained are merely not a reflection of criterion definition and may reflect other factors. For example, race may be a factor. In a study reported by Blazer and colleagues, a higher prevalence was observed among community-dwelling elderly whites compared to blacks [4]. Su and colleagues also reported a low rate of 6% among a community dwelling Chinese elderly population [37]. Another possible factor is the use of hypnotics and sedatives in the general population. We have not systematically examined that factor in this survey but given the widespread use of such medications in the setting of our study [11], it is not implausible to expect that it could have influenced our results. However, the finding of low rates of insomnia is in consonance with our report of low rates of mental disorders in this sample [10] This consistency argues against any notion that the low rates of mental disorders in the sample might have been an ascertainment artifact. Given the simplicity of the questions relating to insomnia and their relative lack of any stigmatizing connotation, it would seem unlikely that answers to them could have been affected by lack of clarity or stigma. We believe, as we have noted before [10], that the age of the sample is likely to be a factor for the low rates. The mean age of our Part II sample was 40.3 (s.d. 0.36) years, reflecting the common age pyramid of a developing country. Since insomnia complaints are more common in middle and old age, the majority of our sample may have not lived through the median age of onset of insomnia complaint of at least two weeks duration.

Unlike commonly reported [16, 22, 31], women were not more likely to have sleep complaints than men. However, even though common, such association is by no means universal [3, 5, 26, 37]. The association of insomnia with age is more consistent, with most studies finding insomnia complaints significantly more among persons aged 65 years and over when compared to those in the 20–40 years age group [20, 32]. Such observations are consistent with the findings in this study. We observed a sustained increase in the prevalence of insomnia with age, with a particularly sharp elevation in risk occurring after the age of 45 years. Consistent with some [7, 8] but not all [3, 6] previous reports, low education was not associated with insomnia in our sample. As noted by Ohayon and colleagues, the association of insomnia with education may be spurious since studies that used multivariate analysis have failed to identify lower education as an independent risk factor [27]. Our findings in regard to the association of insomnia with income are inconclusive, emphasizing the inconsistencies of previous reports in this regard [3, 4, 24, 29]. Consistent with other reports [2, 6, 8, 27, 28] the presence of chronic pain or chronic medical condition significantly elevated the risk of experiencing insomnia. Our findings suggest a particularly high risk among persons with chronic pain, with more than a 3-fold increase in risk of every type of insomnia complaint for such persons. Not surprisingly, having a current DSM-IV disorder constituted a risk for insomnia. Persons with disorders were consistently more likely to report every type of insomnia. The association of mental disorders with insomnia has been highlighted by many previous authors [8, 24, 40].

How much impairment of role functioning can be specifically attributed to insomnia is a question of public health significance. First, the high occurrence of mental disorders, chronic medical conditions, and of chronic pain, each of which can cause role impairment, among sufferers of insomnia confounds its association with disablement. Second, as noted by Ohayon [29], the epidemiology of insomnia is still in its infancy. It is critically important that we better understand its implications for health and functioning in order to estimate its public health burden. In this regard, our attempt at estimating the proportion of role impairment that could be attributed to insomnia represents a novel approach at understanding the condition. Our findings suggest that, unadjusted, persons suffering from any insomnia reported almost 8% decrement in role functioning in the previous month compared to those with no insomnia. A significant proportion (about 23%) of such decrement seemed accounted for by demographic factors as well as by pain, medical, and mental disorders, with pain being the most important factor. However, after adjusting for these factors, about 5.8% decrement in role functioning seemed to be attributable to insomnia. In a cross-sectional design, a direct causal link between insomnia and the level of decrement in functioning noted cannot be firmly inferred. It is also to be noted that other possible confounding factors, including other health conditions not assessed in this survey as well as social problems, could possibly further reduce the extent of the role decrement. Nevertheless, our finding provides a suggestive, albeit indirect, link between the insomnia and functional role impairment. The highly significant decrease in performance that we observed suggests that insomnia, as defined in this report, may be an important public health burden even among a sample drawn from a relatively young population. It is likely that the effect of insomnia on role performance would be higher in samples with a higher proportion of middle-aged and elderly persons, as are likely to be found in most developed countries.