Background
Anxiety and depression symptoms (or psychiatric distress) are more common for those in a disadvantaged socioeconomic position (SEP) [
1‐
4] and inequalities widen with increasing age [
5‐
8]. Gender inequalities, with higher levels of psychiatric distress among adult women, are also a common finding [
9,
10], and these too widen with age [
11,
12]. Gender differences are partially but not completely explained by female socioeconomic disadvantages [
11]. Additionally, psychiatric distress predicts later insomnia symptoms (defined here as trouble initiating or maintaining sleep) [
13,
14], and, vice versa, insomnia predicts later psychiatric distress [
14‐
18], even with adjustment for baseline or historic symptoms. This bi-directional evidence is suggestive of a positive feedback loop where each problem aggravates the other. Indeed, insomnia symptoms exhibit similar social patterning. A female propensity for insomnia symptoms, especially at older ages, has been established in a meta-analysis [
19], and cross-sectional studies on population samples indicate that insomnia symptoms are more commonly experienced by those in a disadvantaged SEP [
20,
21]. Gender differences in insomnia symptoms are attenuated but not fully explained with adjustment for socioeconomic position [
20,
22,
23]. Longitudinal evidence on insomnia is sparse but indicates that women have a propensity to develop insomnia symptoms in middle age and that those in a disadvantaged SEP are more likely to experience chronic, persistent insomnia symptoms than those more affluent [
23]. The mutual association between insomnia symptoms and psychiatric distress may mean the similarities in patterning are not coincidental.
Since insomnia symptoms can come within the range of symptoms that indicate psychiatric distress, it is tempting perhaps to dismiss the social patterning of insomnia symptoms as an expression of inequalities in psychiatric distress. This may be true, but insomnia symptoms can lead to greater distress as well as vice versa. Indeed, trials have shown improved depression outcomes where concurrent insomnia is treated [
24,
25] and some have even called for trials of insomnia treatment to prevent depression [
17,
26]. If inequalities in psychiatric distress are even partially mediated by inequalities in insomnia symptoms then insomnia treatment might have the added benefit of reducing inequalities in psychiatric distress. With this in mind we investigated the extent to which long-term sleep patterns might explain subsequent inequalities in psychiatric distress, taking earlier distress into account. We focus on symptoms in mid-life as this is the life-stage where there is the most widening of socioeconomic inequalities in psychiatric distress [
8], and where women are particularly likely to develop insomnia symptoms [
23].
Results
Descriptive characteristics of the sample (before weighting and imputation) are displayed in Table
1. Psychiatric distress was present in a little more than a quarter of the sample at both ages 36 and 57. The
Healthy sleep pattern was most common, followed by the
Developing and
Episodic Maintenance patterns, whilst
Chronic Mixed was least common.
Table 1
Descriptive data for the analysis variables
Gender | Male | 457 (45.7) |
| Female | 542 (54.3) |
Social class at age 36 | Non-manual | 686 (68.7) |
| Manual | 304 (30.4) |
| Missing | 9 (0.9) |
GHQ (2+) at age 36 | No | 593 (59.4) |
| Yes | 303 (30.3) |
| Missing | 103 (10.3) |
GHQ (2+) at age 57 | No | 706 (70.7) |
| Yes | 255 (25.5) |
| Missing | 38 (3.8) |
Most likely sleep class | Healthy | 464 (46.4) |
| Episodic maintenance | 164 (16.4) |
| Developing | 250 (25.0) |
| Chronic Mixed | 112 (11.2) |
| Missing | 9 (0.9) |
Table
2 displays the results from the first two stages. In model A, as found previously [
23], relative to those with
Healthy sleep patterns, women were more likely to be in the
Developing class than men. In reference to the
Healthy class, those from manual social classes were more likely to be in any of the symptomatic sleep classes than those from non-manual occupations, though this relationship was weak (p = 0.097) for the
Episodic Maintenance class, and strongest for the
Chronic Mixed class. When baseline psychiatric distress was adjusted for in model B, the associations between sleep classes and gender were largely unaffected but social class was no longer significantly associated with membership in the
Episodic Maintenance or
Chronic Mixed classes and the association weakened for the
Developing class (p = 0.051). Respondents with baseline psychiatric distress were generally more likely to be members of symptomatic sleep classes than to be healthy sleepers. However, social class was still important for sleep as an interaction was found whereby those in a manual social class with high baseline GHQ scores were especially likely to be in the
Chronic Mixed class (rather than the
Healthy class). All three sensitivity analyses replicated these findings (see Additional file
1).
Table 2
Odds ratios for sleep class membership with and without adjustment for baseline psychiatric distress
Episodic maintenance class (ref: Healthy) |
Female gender | 0.85 | (0.49-1.47) | 0.81 | (0.45-1.46) |
Manual social class | 1.60* | (0.92-2.78) | 1.55 | (0.65-3.69) |
GHQ (2+) at age 36 | | | 3.57** | (1.82-7.02) |
Social class and GHQ interaction | | | 1.26 | (0.31-5.10) |
Developing class (ref: Healthy) |
Female gender | 4.20** | (2.68-6.60) | 4.02** | (2.52-6.42) |
Manual social class | 1.86** | (1.27-2.72) | 1.73* | (1.00-2.99) |
GHQ (2+) at age 36 | | | 2.32** | (1.26-4.28) |
Social class and GHQ interaction | | | 1.54 | (0.40-5.97) |
Chronic mixed class (ref: Healthy) |
Female gender | 1.36 | (0.78-2.37) | 1.14 | (0.63-2.07) |
Manual social class | 3.34** | (1.69-6.57) | 0.98 | (0.31-3.08) |
GHQ (2+) at age 36 | | | 3.26** | (1.43-7.45) |
Social class and GHQ interaction | | | 9.57** | (2.55-35.95) |
The results from models predicting psychiatric distress at age 57 are displayed in Table
3. In model C, without adjustment for sleep class membership, women and those with high baseline GHQ scores were more likely to have high GHQ scores at age 57. Both of these effects were stronger for those in manual social classes relative to their non-manual counter-parts, though the gender by social class interaction was statistically weak (p = 0.072). Model D adjusted for sleep class membership. All of the symptomatic sleep classes, but especially the
Chronic Mixed class, were associated with higher odds of psychiatric distress at age 57 than in the
Healthy sleep class. The gender and baseline GHQ effects were attenuated, but only the gender difference became non-significant. The interaction between social class and baseline GHQ was also attenuated into non-significance, but the interaction between social class and gender was actually stronger with adjustment for insomnia symptoms, passing the p < 0.05 level.
Table 3
Odds ratios for psychiatric distress at age 57, with and without adjustment for insomnia symptoms
Female gender (ref: Male) | 1.45** | (1.01-2.09) | 1.24 | (0.79-1.97) |
Manual social class (ref: Non-manual) | 1.03 | (0.57-1.87) | 0.92 | (0.47-1.78) |
GHQ (2+) at age 36 (ref: <2) | 2.20** | (1.55-3.13) | 1.69** | (1.16-2.47) |
Gender and Social class interaction | 2.00* | (0.96-4.14) | 2.16** | (1.05-4.46) |
Social class and GHQ Interaction | 2.19** | (1.08-4.45) | 1.51 | (0.71-3.20) |
Episodic maintenance class (ref: Healthy) | | | 2.90** | (1.35-6.25) |
Developing class (ref: Healthy) | | | 4.09** | (2.36-7.09) |
Chronic mixed class (ref: Healthy) | | | 9.23** | (5.10-16.68) |
The sensitivity analyses for models C and D produced broadly consistent findings (details in Additional file
1). The
Episodic Maintenance,
Developing, and
Chronic Mixed classes (relative to the
Healthy class) were associated respectively with mean differences of 1.04, 1.36 and 2.16 GHQ points at age 57 (standard errors were 0.30, 0.24 and 0.41), adjusting for the other variables in the model (including baseline GHQ scores).
Discussion
By using 20 years of longitudinal data this paper goes further in demonstrating the inter-relatedness of insomnia and psychiatric distress than is possible with cross-sectional research or longitudinal research that only utilises cross-sectional (e.g. baseline) measures of insomnia symptoms. Even after adjustment for baseline psychiatric distress middle-aged women experienced greater odds of developing insomnia symptoms in late middle-age than men, and those who experienced psychiatric distress and socioeconomic disadvantage were more likely than others to be experiencing chronic insomnia symptoms. Psychiatric distress was more likely to recur or still be present 20 years later for those in socioeconomic disadvantage compared to those more affluent, but this association was reduced to non-significance when conditioning on intervening insomnia symptom trajectories. Irrespective of earlier psychiatric distress, women were more likely than men to experience psychiatric symptoms around age 57, though women from a manual social class were particularly disadvantaged in this respect. The greater likelihood of insomnia symptoms developing in middle age among women compared to men explained the overall association between gender and psychiatric distress at age 57, but not the extra disadvantages for women in a manual social class.
The findings concur with previous research showing that psychiatric distress and insomnia symptoms occur more frequently among women and those in socioeconomic disadvantage [
2,
3,
19]. Cross-sectional evidence shows that sleep quality can mediate associations between socioeconomic disadvantage and poor mental health [
37,
38], but these findings show that this can be true over the long-term, up to 20 years, and that gender differences in mental health can also be mediated by sleeping problems. The findings agree with evidence that socioeconomic inequalities in psychiatric distress are at least in part due to symptoms being more persistent among the disadvantaged [
3,
39,
40], and these results show that chronic insomnia symptoms are associated with socioeconomic inequalities in persistent distress. Research showing that more severe insomnia, including problems with both sleep maintenance and initiation, is associated with more severe psychiatric symptoms was also supported [
41], and the findings add that this is particularly true for those who experienced long-term problems with both sleep maintenance and initiation.
It is clear from this study that insomnia symptoms and psychiatric distress are strongly associated with each other over the long-term, and that the social patterning of these problems is not independent. The propensity for those in socioeconomic disadvantage to experience persistent or recurring psychiatric distress is strongly associated with the propensity of these individuals for chronic insomnia symptoms, and the propensity for women to experience psychiatric distress in late-middle age is strongly associated with their propensity to develop insomnia symptoms around this time. Poor sleep appears to be characteristic of the type of psychiatric distress which is experienced more often by women and those in socioeconomic disadvantage. This suggests that treatment of insomnia has potential value for alleviating inequalities in psychiatric distress. Inequalities in psychiatric distress might also be helped by tackling the structural or societal constraints that make healthy sleep more difficult for particular groups [
42]. The only element of the social patterning of psychiatric distress which was not strongly associated with insomnia symptoms in this study was the particular tendency for women in socioeconomic disadvantage to have high GHQ scores, and so such interventions would be limited in the extent to which they could alleviate this inequality.
Insomnia symptoms are thought to precipitate poor mental health under a diathesis-stress model [
43] where the stress and opportunities for negative rumination associated with long, wakeful nights facilitate expression of a latent vulnerability to psychopathology [
16,
41]. Insomnia with short sleep duration can also impact on neuro-cognitive performance (e.g. attention switching, visual memory, processing speed) [
44], and such performance deficits could be distressing to experience. Conversely, anxious, negative rumination may make it more difficult to sleep [
16,
41]. A diathesis-stress model is also often used to account for socioeconomic inequalities in distress [
43], with the accumulation of stressful circumstances and low levels of coping resources associated with socioeconomic disadvantage explaining the higher levels of psychiatric morbidity, rather than differences in underlying vulnerability between social strata [
45,
46]. Socioeconomic disadvantage has additionally been posited as restricting the autonomy required to create ideal conditions for sleep [
42]; recommendations such as not taking your problems to bed, for example, could be especially difficulty for those with more problems due to their disadvantaged circumstances, or those in lower status occupations may be more likely to experience disruptions to sleeping patterns from shift work. The inter-play of these processes could easily result in the findings reported here, which suggest a clustering of co-morbid persistent psychiatric distress and chronic insomnia symptoms among those in socioeconomic disadvantage.
As socioeconomic factors partially explain gender differences in psychiatric distress and insomnia symptoms [
11,
20,
22], it is plausible that similar processes partially account for these differences. However, as in previous research, there were still independent effects of gender in this study even after conditioning on social class, and so further explanation is needed beyond gender inequities in the socioeconomic distribution. One hypothesis is that women experience additional stress from domestic and caring roles, over and above roles in employment or elsewhere, but studies have failed to show that this substantially accounts for gender differences in psychiatric distress or insomnia [
22,
47,
48]. Female sex may also constitute a biological diathesis, with sex differences having been shown in a number of biological processes connected with depression [
49]. This would lead to stronger gender differences where stress levels were also high, such as in a disadvantaged SEP, as was found in this study. A biological process uniquely experienced by women and particularly relevant to the age-group under study here is the menopausal transition, but evidence is mixed as to associations between menopause and depression or insomnia symptoms [
50‐
52], and suggests that links could be accounted for by other life experiences common to women of this age in this cultural context such as the onset of chronic conditions, or children leaving home [
53,
54]. If some of these life experiences were more common amongst people living in socioeconomic disadvantage, then this could also explain the interaction with social class.
The findings support the idea of a positive feedback loop where insomnia and psychiatric distress aggravate each other [
16,
41]. There are nevertheless some limitations to the causal inferences that can be made, considering that the measure of sleep patterning includes indicators of insomnia symptoms at ages 36 and 57 (i.e. concurrent with the measures of psychiatric distress). The insomnia symptoms expressed in the
Chronic Mixed or
Episodic Maintenance patterns tended to be present at baseline and therefore probably began earlier so it is not clear whether they preceded or were preceded by psychiatric distress or socioeconomic disadvantage. Only with the
Developing pattern is there evidence that earlier psychiatric distress predicts the later development of insomnia symptoms. Gender, taken here as an indication of biological sex, probably precedes insomnia symptoms and psychiatric distress (implying causal direction), both because biological sex is usually constant over the lifecourse, and because gender was primarily associated with symptoms that developed post-baseline. Symptomatic sleep patterns did predict distress at age 57, but were also predicted by distress at age 36, so the link may be associative rather than causal. Questions about causal direction between sleep and psychiatric distress have been addressed elsewhere, tending to show relationships in both directions [
13‐
15,
17,
18,
41], but the main focus here was on the social patterning. Additionally, considering the five year gaps between measurements, apparently persistent symptom patterns may represent recurring rather than persistent experiences. A further limitation is that people were only observed between ages 36 and 57; date from the USA suggests that the prevalence of sleep disturbance rises between ages 45–59 in men and women, but declines at older ages [
55], so the symptoms observed in this cohort may not persist into later life. However, our previous research found higher levels of insomnia symptoms in the older cohort of the Twenty-07 study as they aged from 56–76 years [
23].
Competing interests
MJG and MB declare that they have no competing interests. CAE declares that he has received payment for other work from Boots UK, Novartis, UCB pharma, and Constable & Robinson; has received grant funding from the NIH, the Chief Scientist Office in Scotland and Breast Cancer Research; and is a co-founder and shareholder of Sleepio Ltd.
Authors’ contributions
MG designed the study, performed the statistical analysis and drafted the manuscript. CE and MB participated in the design of the study and helped to draft the manuscript. All authors read and approved the final manuscript.