Bipolar disorder is potentially a lifelong episodic and recurrent illness characterised by prominent and persistent mood swings and with a diverse course that can often result in significant distress and impairment in functioning. Sleep disturbances are central to the onset, relapse, recovery, prognosis and outcome of bipolar disorder.¹ Sleep disturbances occur in all stages of bipolar disorder and play an essential part in the aetiology and maintenance of the disorder including periods of remission.²

Contrary to previously held assumptions, remission periods (euthymia) in bipolar disorder are not asymptomatic periods. Such euthymic periods are characterised by psychopathologies including sleep disturbances and poor sleep quality. Such psychopathologies and disturbances produce a more severe clinical outcome that considerably affects the quality of life of affected individuals.^3,4,5

Poor sleep quality in bipolar disorder causes substantial medical and psychological morbidities such as reduced global functional ability, relapses, recurrences, neurocognitive impairment and poor prognosis.^6,7,8,9 Poor sleep quality also increases the risk of cardiovascular morbidity (e.g. heart attack), diabetes mellitus, cerebrovascular accidents (strokes), obesity, neurocognitive impairments, unintentional injuries (such as motor vehicle accidents, falls, poisoning, fire/burns, and increased workplace accidents).^10,11,12,13 Poor sleep quality negatively affects daytime functioning, social functioning, occupational functioning, and other important areas of functioning.^14,15

Very few studies have been conducted on sleep quality during the euthymic phase of bipolar disorders. Also, very little is known about the correlates and predictors of sleep quality during this period.¹⁶ The majority of such existing studies have been carried out in non-Caucasian populations. This is concerning since racial/ethnic disparities exist for sleep quality. Furthermore, there is evidence to suggest that sleep quality amongst racial/ethnic minorities is disproportionately understudied.¹⁷

Understanding the factors associated with sleep quality during the euthymic phase of bipolar I disorder may play a significant role in improving clinical course and help to negotiate a more favourable outcome for patients with bipolar disorder.

This study aimed to assess sleep quality and its correlates amongst well-characterised euthymic patients with bipolar I disorder.

The Mulberry study is a cross-sectional study comparing sociodemographic and clinical characteristics such as neurocognitive function, psychopathology, sleep, physical activity and spirituality between patients with bipolar disorder, patients with schizophrenia and healthy controls.¹⁸ A complete account of the methodology has been described elsewhere.¹⁸ The current analysis is restricted to participants with bipolar disorder in the Mulberry study. The study was carried out in a teaching hospital, and state hospital, in Ibadan, Nigeria. Assessments were carried out between February and October 2018 on 76 consecutive and consenting participants with bipolar disorder, aged between 18 and 60 years who were in remission.

A total of 110 participants with a diagnosis of bipolar I disorder were recruited into the study, however, only 76 met the criteria for euthymia. The 76 participants (47 females, 29 male) were subsequently included in the current analysis. The mean age was 39.09 ± 11.10 years. A total of 37 (48.7%) participants had poor quality sleep during the past 1 month. There was no significant difference in socio-demographic and clinical characteristics between the group with poor sleep quality and the group with good sleep quality except for marital status and lifetime suicidal plan. A significantly higher proportion of the participants who had poor sleep quality were single or never married compared to those with good sleep quality (43.2% vs. 26.3% p = 0.013). Also, a significantly higher proportion of participants with poor sleep quality had made a suicide plan compared to those with good sleep quality (24.3% vs. 7.7% p = 0.047) (see Tables 1a and 1b).

Although the differences were not statistically significant, participants with poor sleep quality had a higher average length of an episode, used higher doses of mood stabiliser and antipsychotics per day, had lower social and occupational functioning, lower monthly income and financial net worth than participants with good sleep quality (Tables 1a and 1b).

Nearly half (48.7%) of the euthymic bipolar I patients had poor sleep quality suggesting that many euthymic patients with bipolar disorder display substantial disturbance in sleep quality during remission. This means that about half the population of bipolar disorder patients purportedly in remission are at the risk of neurocognitive impairment, low immunity, obesity, heart disease, diabetes, infertility, poor social and occupational functioning and other effects of poor sleep quality because of impaired sleep quality.^10,11,12,13 Sleep disturbance is also a trigger for relapses and recurrences. This can lead to a vicious cycle of relapses and poor sleep quality. The result adds to existing reports indicating that euthymic bipolar patients have substantial poor sleep quality even when they are adjudged symptomatically to be in remission.^{15,23,24,25,26} The prevalence of sleep quality in our study (48.7%), is lower than those of existing studies in similar euthymic patients (56.5% – 82.9%).^23,27,28 Methodological differences could have accounted for the lower prevalence observed in our study.

We found that a significantly higher proportion of the participants who had poor sleep quality were single or never married compared to those with good sleep quality. This is in keeping with existing studies showing that poorer sleep quality is more commonly associated with unpartnered relationship status (single, divorced, cohabiting, widowed) than partnered relationship status (i.e. married or cohabiting).^29,30 Nevertheless, marital happiness and anxiety are confounders in the associations between sleep quality and unpartnered relationships. Marital happiness is associated with a better quality of sleep in women,^31,32 while anxiety in close relationships is associated with poorer subjective sleep quality.^33,34 Such associations suggest that being in a partnered relationship may be beneficial to the health and well-being of patients with bipolar disorder.

We found a significant association between sleep quality and a suicidal plan. This is in keeping with existing studies that found an association between suicidal behaviours and impaired quality of sleep amongst euthymic bipolar patients.^35,36 Existing studies indicate that there is an association between impaired sleep quality and suicidal ideation, suicidal attempts, and completed suicide.³⁷

Furthermore in males, poor sleep quality is associated with suicide attempts, but not in females. This implies that there may be sex differences in the relationship between disturbed sleep quality and suicidal behaviour. Testosterone levels and age have been implicated in this phenomenon.^38,39 It has been suggested that low testosterone level is associated with suicide in the elderly, while high testosterone level is associated with suicide in adolescents and young adults.⁴⁰

The mean sleep latency in participants with poor sleep quality in our study (25 min) was significantly higher than that for participants with good sleep quality (15 min). Additionally, it was higher than the normal 10–20 min usually found in adults. The findings regarding sleep latency are in parity with existing reports that indicate that euthymic bipolar patients have longer sleep-onset latency than during manic or depressive episodes.⁴¹

We found evidence of a significant association between poor sleep quality and subjective quality of sleep, use of sleep medication and daytime dysfunction amongst euthymic patients with bipolar disorder. Notably, these associations remained significant after controlling for several covariates in binary logistic regression.

The finding that daytime dysfunction was independently associated with euthymic bipolar patients replicates the report on sleep by Geoffroy et al. conducted amongst euthymic bipolar disorder patients and healthy controls. The authors found poorer sleep efficiency, frequent sleep disturbances and daytime dysfunction to be associated with euthymic bipolar disorder.⁴² Similarly, Cretu et al.⁴³ found that bipolar patients in remission compared to healthy controls had significantly worse daytime dysfunction. Earlier studies also showed similar trends, for example, Harvey et al.²⁵ showed that daytime dysfunction was poorer in euthymic bipolar patients compared to healthy controls.

Poor sleep quality in euthymic bipolar disorder patients was independently associated with medication use. This is in keeping with existing studies showing that euthymic patients with bipolar disorder are more likely to use sleep medications compared to healthy controls.⁴³

There were certain limitations to this study. First is the use of PSQI which is both subjective and retrospective and may be laden with the problem of recall bias. Second, the PSQI generates a global score from its seven component scores. These components are influenced by many factors. Third, the sample size was small; this may have reduced the power of the study to detect significant differences.

Amongst euthymic bipolar patients, poor sleep quality frequently persists during euthymic periods and can be a focus of targeted intervention. An assessment of sleep quality should be routinely carried out in the assessment of euthymic bipolar patients.