tion in BD as they persist in remission, appear to be heritable,
and hence co-segregate within families [
46
,
47
]. Therefore,
our study provides further evidence that working memory
could serve as a potential neurocognitive endophenotype in
remitted BP.
Association between cognition and clinical variables
Regarding the clinical characteristics of our bipolar sam-
ple, we found no relationship between test measures and
disease characteristics indicating severity of the disorder
(e.g. number of episodes, age of onset, bipolar type). How-
ever, subclinical mood symptoms and sleep disturbances
were associated with cognitive speed, attention, and verbal
memory. Although patients with bipolar disorder have his-
torically been characterised as returning to full remission
between affective episodes, recent studies demonstrated
that some residual symptoms may persist [
48
,
49
]. It has
been previously demonstrated that subthreshold mood
symptoms are associated with impairments in attention
and verbal memory [
11
,
50
–
52
]. In a study by Bonnin et al.
[
53
], the degree of cognitive functioning was best predicted
by subdepressive symptoms, and a meta-analysis reported
that more rigorously defined euthymia goes along with
smaller cognitive impairments [
2
]. Therefore, it seems that
the neuronal circuitry activated in the modulation of emo-
tions overlaps with brain regions involved in neurocogni-
tion. Besides subclinical depressive symptoms, 37 % of our
remitted bipolar sample reported sleep disturbances like
initial or middle insomnia or an abnormal increased sleep-
ing time. Rates of sleep disturbances are usually very high
in remitted BP [
54
,
55
], and BD is highly associated with
sleep and circadian rhythm abnormalities [
49
,
56
,
57
]. It is
well known that sleep deprivation in healthy people causes
cognitive deterioration [
58
,
59
]. Furthermore, it has been
demonstrated that sleep and circadian rhythms are involved
in the cognitive deficits in BD through overlapping neuro-
biological systems [
60
]. In our bipolar sample, sleep distur-
bances were associated with impairments in psychomotor
speed, divided attention, and verbal learning. In a recently
published study, we reported accordingly that several cog-
nitive impairments in euthymic BP were predicted by sleep
disorder [
11
]. Taken together this underscores the need
for more extensive research on the influence of sleep dis-
turbances on cognitive deficits in BD. In conclusion, our
results suggest that residual symptoms negatively affect
cognitive functioning in BD. Therefore, an optimised
antidepressant treatment and sleep regulation should be
enforced in patients complaining about cognitive impair-
ments. Even though our results do not disprove the idea of
neurodegenerative processes in BD [
1
], our data underscore
the importance of residual symptoms. This also has impli-
cations for future research and calls for the improvement of
treatment options for cognitively impaired BP.
