Study data published in Psychological Medicine suggest that sleep problems may affect cognition in patients with bipolar disorder. However, physical activity, body mass index (BMI), and circadian rhythm had no apparent relationship with cognition.
Patients with bipolar disorder (n=46) were recruited from research databases, patient support groups, and National Health Service programs in northeast England. Age- and sex-matched healthy controls (n=42) were recruited from local volunteer and healthcare organizations. Participants underwent a battery of neuropsychiatric evaluations intended to measure reaction time, processing efficiency, attention, and perceptual speed. Ex-Gaussian analyses were used to calculate the distributions of patient reaction time. Participants were also instructed to wear a triaxial accelerometer for 21 days, during which they also completed a sleep diary. “Normal” sleep was defined as 6 to 10 hours of sleep per night, with a regular sleep-wake cycle and no evidence of sleep apnea. Hierarchical regression analyses were conducted to assess the relationship between cognitive task performance and sleep.
Patients and controls had comparable demographic characteristics. Mean patient age was 46.8±11.1 years; mean control age was 42.5±11.9 years. The majority of all participants in both groups were women (69% and 67.4%, respectively). Both bipolar disorder type I and type II were represented.
Compared with controls, patients with bipolar disorder had significantly higher depression scores, higher BMIs, and were less likely to be in education or employed. Overall, 16 patients with bipolar disorder had “normal” sleep, while the remaining 28 were classified as “abnormal.” In hierarchical regression models, age, intelligence quotient, and depression symptoms explained a significant proportion of the variance in the outcome measures for each cognitive task. Additionally, the presence of a sleep abnormality was significantly associated with higher intra-individual variability of reaction time distributions on the Attention Network Task. However, physical activity, BMI, and circadian rhythm did not further explain variance in outcomes for any task.
These data suggest that sleep abnormalities and cognition may be closely related in patients with bipolar disorder. Study limitations include the small cohort size and exploratory design. Additionally, analyses were not sufficiently powered to detect small between-group differences.
“The relationship between these factors is complex and requires further research in patient populations with larger sample sizes,” investigators concluded. “Understanding the factors associated with poor cognition in [bipolar disorder] opens up options for interventions to enhance cognition and ultimately the function and quality of lives of patients.”
Burgess JL, Bradley AJ, Anderson KN, Gallagher P, McAllister-Williams RH. The relationship between physical activity, BMI, circadian rhythm, and sleep with cognition in bipolar disorder [published online June 29, 2020]. Psychol Med. doi: 10.1017/S003329172000210X