Demographic and Clinical Characteristics Across Entire Sample
As evident in , approximately half (55.9%) of the sample was female, the majority was Caucasian, and the mean age was 38.0 (SD = 13.1) years. Mean age at onset for bipolar illness was 16.8 years (SD = 8.8). In the past year, 26.9% of participants exhibited a rapid cycling course. With respect to clinical status, the majority of participants were syndromally depressed (36.1%) or euthymic (36.0%), followed by subsyndromal depression (13.9%), syndromal elevation (6.9%) and subsyndromal elevation (0.6%).
Demographic and Clinical Characteristics of Short Sleepers (SS), Normal Sleepers (NS), Long Sleepers (LS), and Total Sample
Demographic and Clinical Characteristics among Sleep Duration Groups
As evident in , 641 participants (31.7 %) were classified as short sleepers (SS), 760 participants (37.5 %) as normal sleepers (NS), and 467 participants (23.1 %) as long sleepers (LS). A total of 156 (7.0%) participants did not meet criteria for any of the three sleep duration groups (i.e., either had missing data or did not fit cutoffs for any of the sleep duration groups) and were retained in analyses based on the entire sample but excluded from analyses involving SS, NS and LS groups. The three sleep duration groups were similar with respect to gender and differed significantly in ethnicity, age at study entry, age at illness onset, illness duration, rapid cycling and clinical status. Specifically, although the three groups were predominantly Caucasian, the SS group was comprised of fewer Caucasian people (75.9%) than both the NS (82.5%) and LS (84.4%). Also, SS were younger than LS. SS and LS had an earlier age of onset compared to NS, though SS and LS did not differ from each other. SS also had a longer illness duration relative to both NS and LS. NS and LS did not differ from one another in illness duration. SS had a higher percentage of rapid cyclers in the past year relative to both NS and LS. LS had a higher percentage of rapid cyclers relative to NS.
illustrates the differences in clinical status across the three sleep duration groups. Planned follow-up analyses examined group differences among individual clinical status categories with a Bonferroni correction (adjusted p-value =.003). Results suggested that SS were the most symptomatic, followed by LS. SS tended to have a greater frequency of syndromal elevation compared to both LS and NS. Both SS and LS were more frequently syndromally depressed than NS, though SS and LS did significantly differ. NS were more frequently euthymic compared to both SS and LS. However, SS were less frequently euthymic than LS. No significant differences were obtained for subsdyndromal depression or subsyndromal elevation.
Sleep Duration and Mood State at Entry to STEP-BD.
With respect to bipolar subtype, the majority of patients were diagnosed as BP I (62.4%), followed by BP II (25.8%) and BP NOS (7.9%). TST was significantly different among the three bipolar subtypes, F(2, 2,018) = 3.36, p < .05. Specifically, TST was higher in BP I (M = 7.38, SD = 2.59) and BP II (M = 7.31, SD = 2.30) than BP NOS (M = 7.06, SD = 2.27) subtypes. There were no significant differences for sleep variability, F(2, 2,023) = 0.14, p = 0.87, minimum hours of sleep, F(2, 2,023) = 2.77, p = 0.06, maximum hours of sleep, F(2, 2,023) = 2.23, p = 0.11, or sleep duration group, χ2 =5.67, p = .23.
Sleep Functioning Parameters among Sleep Duration Groups
Means for the sleep functioning parameters are presented in . Across the entire sample, average TST was 7.28 h (SD = 2.48), and sleep variability was 2.78 h (SD = 3.02), minimum sleep was 5.90 h (SD = 2.68), and maximum sleep in the past week was 8.66 h (SD = 3.12). As expected given the inclusion criteria for our sleep duration groups, the three groups differed significantly in TST, minimum sleep, and maximum sleep. Furthermore, LS displayed significantly higher sleep variability, relative to both the SS and NS. When age and ethnicity were included as covariates, all results remained the same.
Sleep Functioning Parameters across Short Sleepers (SS), Normal Sleepers (NS), Long Sleepers (LS), and Across the Total Sample
Sleep Duration Groups and Associations with Symptom Severity
As evident in , SS exhibited higher mania severity scores on the YMRS relative to both LS and NS. LS and NS did not differ from each other. SS exhibited higher symptoms of depression on the MADRS compared to both LS and NS, whereas LS exhibited higher depressive symptoms than NS. SS were significantly higher on severity of anxiety from the CMF compared to both NS and LS. NS and LS did not significantly differ from one another on anxiety severity. While SS scored higher on irritability from the CMF than NS, SS did not differ significantly from LS. Results did not change when age and ethnicity were included as covariates, with the exception of irritability that no longer significantly differed among the sleep duration groups, F(1, 718) = 1.15, p = 0.31.
Symptom Severity, Functional Status, and Life Satisfaction across Short Sleepers (SS), Normal Sleepers (NS), Long Sleepers (LS) and the Total Sample.
Given the construct overlap between our sleep disturbance groups and individual sleep items on measures of mania (YMRS Item #4) and depression (MADRS Item #4), we re-ran the above analyses with these individual sleep items removed. Results regarding sleep disturbance and mania symptom severity did not significantly change. Removing the sleep item from the MADRS, however, revealed that SS no longer significantly differed from LS in depression symptoms (p = 0.13).
Sleep Duration Groups and Associations with Function and Quality of Life
On measures of function, both SS and LS generally exhibited poorer function than NS. On the GAF, SS and LS exhibited lower scores in global functioning compared to NS. SS and LS did not significantly differ from one another on the GAF. On the LIFE-RIFT, SS and LS again exhibited greater impairment than NS on the total LIFE-RIFT score and the work, recreation and satisfaction LIFE-RIFT domains. SS and LS did not differ significantly from one another on these measures. On the relationship domain, SS exhibited higher impairment than NS and LS. After controlling for age and ethnicity, NS and LS no longer significantly differed from one another in relationship impairment. With respect to quality of life, SS and LS had poorer life satisfaction compared to NS, as indicated by both the overall and total score on the Q-LES-Q. SS and LS did not significantly differ on the Q-LES-Q. Controlling for age and ethnicity did not significantly change the results.
We further examined sleep functioning within the euthymic patients to understand sleep disturbance during the inter-episode period. For euthymic patients (n = 750), average TST was 7.53 (SD = 1.73), maximum hours of sleep was 8.57 (SD = 2.04), minimum hours of sleep was 6.49 (SD = 2.01), and sleep variability was 2.10 (SD = 2.11). The proportion of euthymic patients classified as SS (34.6%), NS (38.0), and LS (27.4%) paralleled that across the entire sample. Paired samples t-tests indicated that a higher frequency of euthymic patients were classified as NS (38.0%) relative to both SS (34.6%) and LS (27.4%) (ps < .001). For euthymic patients, the three sleep duration groups did not significantly differ with respect to life satisfaction or total Q-LES-Q scores (ps > .50). For function, the three sleep duration groups did not differ on the GAF (p = .17) but differed on the LIFE-RIFT total score, F(2, 213) = 3.65, p < .05. Follow-up pairwise comparisons indicated that the SS had greater life impairment than NS (p < .05) but that none of the other sleep duration groups significantly differed.