Evaluating sleep in bipolar disorder: comparison between actigraphy, polysomnography, and sleep diary

Bipolar disorder is a severe, recurrent mental illness affecting 1–4% of the population (1). Characterized by episodes of mania, hypomania, and often depression, bipolar disorder frequently includes both sleep disturbance (2, 3) and circadian disruption (4–6). Reduced need for sleep and insomnia/hypersomnia are common symptoms of the manic and depressive phases of bipolar disorder, respectively. Furthermore, sleep disturbance persists in the period between episodes (the inter-episode period) and may be a mechanism contributing to illness relapse. For example, one study found that 70% of an inter-episode bipolar disorder group had a clinically significant sleep problem, and 55% met diagnostic criteria for insomnia while inter-episode (7). A second study revealed that sleep disturbance is the most common prodrome of mania and the sixth most common prodrome of bipolar depression (8). Furthermore, a prospective study utilizing daily monitoring reported that changes in self-reported sleep duration (change in total sleep time > three hours) was strongly suggestive of an impending mood episode (9). Though it is unclear as yet whether sleep disturbance is a symptom of current episodes, prodromal to emerging episodes, or a mechanism of future illness relapse, taken together these results underscore the importance of monitoring sleep in bipolar disorder. Actigraphy has proven to be a useful tool for estimating sleep and daily activity patterns (10), but has not been systematically evaluated in bipolar disorder. Actigraphy is a small wristwatch-like device containing an accelerometer that records and stores motion, from which periods of sleep and wakefulness may be inferred. Though polysomnography (PSG) is a widely accepted objective measure of sleep, actigraphy has several advantages over PSG in estimating sleep: it is relatively inexpensive, minimally disruptive, and can be worn continuously in ecological environments for periods of one month or more. When compared to daily sleep diaries, actigraphy is again preferable insofar as it is less burdensome to participants and not subject to the recall biases of self-report estimates (11). Validation work with healthy, non-disordered adults has established an encouraging 90% concordance between actigraphy and PSG (12, 13). However, there are at least two reasons to suspect that actigraphy estimates may be less accurate in bipolar disorder. First, actigraphy appears to be less accurate in populations with fragmented sleep (14) and in periods of quiet wakefulness, such as the sleep onset period (15). Numerous studies have documented that actigraphy has a tendency to overestimate total sleep time and underestimate wake during sleep in insomnia (15–18). As over half of individuals with bipolar disorder meet diagnostic criteria for insomnia, and many more demonstrate other types of sleep disturbance or subclinical insomnia, it is unclear to what extent actigraphy may similarly exhibit underestimation of wakefulness in a bipolar population. Second, numerous medications for bipolar disorder have sedative properties which may lead actigraphy to overestimate sleep (based on reduced mobility) in this population. Hypnotic use has been shown to yield overestimates of actigraphically-determined sleep in patients with chronic insomnia (15), adding support to this potential limitation. A handful of studies have successfully used actigraphy to document sleep and circadian disturbance in individuals with bipolar disorder (3, 7, 19, 20) and in those at elevated risk for bipolar episodes (21). However, without systematic comparison to PSG or sleep diaries, the interpretability of these actigraphy findings is unclear. Moreover, as sleep diaries are often a standard assessment tool by which sleep disturbance is evaluated (22), it is important to compare actigraphy estimates against both PSG (a similarly objective measure of sleep) and sleep diaries (a subjective measure of sleep) to determine its utility in bipolar sleep monitoring. The aims of the present research were twofold. First, concordance rates between PSG, actigraphy, and sleep diary were compared between diagnostic groups. Based on prior insomnia validation work, Aim 1 tested the hypothesis that concordance between actigraphy, PSG, and diaries would be lower in individuals with bipolar disorder than in a matched control group free of lifetime psychiatric illness. Aim 2, included on an exploratory basis, was to systematically evaluate the effects of insomnia presence and psychotropic medications on actigraphy concordance, to evaluate the utility of actigraphy in bipolar sleep monitoring. The present research focused on the inter-episode period given the potential for early detection of illness episodes via changes in sleep.