The strong negative association between VI
and self-rated suicidal thinking or wish to die, and its positive association with self-rated vital drive or wish to live suggest that VI
may be associated specifically with contrasting forces that often confront one other in mood disorder patients–the wish to die and the wish to live 
. The lack of correlations between VI
and other subjective phenomenological measures, including depressed mood, dysphoria, anxiety, unusual body-perception experiences (cœnesthesias), and autonomic-neurovegetative symptoms suggests at least two levels of subjective psychopathology in depression: [a] experiences of decreased fundamental vitality rooted in psychomotor activity (reflected in VI
scores), and [b] more cognitive experiences of hopelessness-helplessness (self-rated depression) or of anguish, fear, anger and distress (self-rated anxiety and dysphoria) associated with abnormal somatic experiences (self-rated autonomic-cœnesthetic symptoms). A similar perspective on descriptive mapping of human emotional life into levels of particular feeling-states and emotions was articulated nearly a century ago by Max Scheler 
. Such an approach, with a focus on specific elements of emotional life, might provide a useful theoretical framework for future research, including psychobiological studies, in mood disorders 
In spite of comparable levels of clinician-rated depression across diagnoses, there were relevant differences in depressive phenomenology in bipolar disorders versus unipolar major depressive disorder subjects. In particular, the higher scores for self-rated suicidal ideation found in bipolar-II disorder subjects accord with recent reports of high risks of suicide and attempts in both bipolar I and II disorder patients, especially in relation with intense impulsivity and dysphoria 
. The greater prevalence of clinician-rated dysphoria in the bipolar-II subjects versus those with either bipolar-I or unipolar major depressive disorder also accords with the same view of type-II bipolar disorder as an illness underlined by impulsive-emotional dysregulation and possible relations with other clinical syndromes characterized by impulsive-emotional dyscontrol 
In addition, bipolar-I disorder subjects scored significantly lower in self-rated vital drive or wish to live than unipolar major depressive disorder subjects. This observation of a diminished self-perceived level of vigor and vital drive, along with decreased motility, confirms the greater likelihood in bipolar depression of psychomotor retardation, lethargy, and lack of vitality compared to unipolar depression 
. Such phenomenological differences between unipolar and bipolar depressive episodes may be related to neurobiological mechanisms including those underlying attentional control, visuospatial and sensory processing, and emotional regulation 
, differences in central monoaminergic systems 
, and altered electroencephalographic patterns during sleep phases 
In light of the findings reported, we also hypothesize that theoretical models for suicide research and prevention might profit by shifting focus from monitoring of experiences of disturbed affect and emotions to consideration of fundamental aspects of vitality and psychomotor activity. Relevant considerations might include two crucial clinical phenomena, potentially emerging during major depression and long recognized in classical psychopathology as associated with increased vulnerability to suicide. The first phenomenon is a time during the process of recovery from depression when psychomotor inhibition resolves before depressed mood. This view predicts that suicidal impulses are more likely to be acted upon at the particularly dangerous time of partial recovery 
. The second phenomenon involves the presence of mixed affective states during bipolar depressive phases that can create a highly explosive mixture of deep sadness, rage and despair as well as psychomotor arousal associated with impulsive aggression leading to self-harm or death 
Recent research on the neurobiology of suicidal behaviors indicates that impulsive aggression as well as impaired judgment or deficient risk assessment may constitute core endophenotypes of suicidality 
. In addition, an aggressive diathesis can be viewed as an imbalance between the diminished effects of descending control systems in the orbital frontal and anterior cingulate cortices, and excessive emotional and aggressive drives arising from limbic elements including the amygdala and insula 
. The modulation or suppression mechanisms of aggressive behavior with negative consequences provided by descending control systems in the prefrontal cortex are both influenced by predicting expectations of reward and punishment as well as by early sensory and social information processing, and early cognitive appraisal 
In light of this theoretical model of suicidal process we might hypothesize that a sudden increase of subjectively perceived wish to die might serve more as a provoking trigger stimulus of self-aggression. However, a chronic and sustained self-perceived lack of vigor and vitality might rather interfere with the mechanism of aggressive diathesis as an influencing factor in early processing of other provoking trigger stimuli.
The suggested association of VI with an inner balance between subjective experiences of a wish to live versus a wish to die might represent a complementary factor to be assessed and monitored clinically alongside various other components and risk factors for vulnerability to suicidal behavior. The need for better prediction of suicidal risk is particularly important for evaluating persons with a bipolar disorder, with predominantly dysphoric longitudinal morbidity and with impulsive aggressive personality traits, as well as those with psychotic mood disorders which also involve impaired risk-assessment and enhanced proneness to violent behavior.
Bipolar mood disorders are often characterized by particularly severe, unstable, and frequently changing, long-term morbidity leading to functional impairment and increased mortality. Therefore, rational and safe clinical management of depressed patients requires timely differentiation of bipolar and unipolar mood disorders. In particular, it is challenging to differentiate bipolar from unipolar disorders when early episodes present as major depression and a history of mania or hypomania is not available or not recognized, as is particularly likely in bipolar-II disorder patients. We suggest that biological measures such as VI might help to limit misdiagnosis and delayed or inappropriate treatments.
Limitations of this study include sample size that was small but sufficient to demonstrate a relationship of VI
to suicidal ideation and to diagnosis (). Although it is acknowledged that a small sample size increases the risk of type II error and a power analysis was not conducted prior to this proof of concept study, a post-hoc analysis found statistical power to be adequate. In addition, actigraphy monitoring needs to be repeated through various morbid states and in subjects with versus without previous suicide attempts. However, our previous research has shown that VI
has both stable (trait-like) and varying (state-like) features 
. It is possible that the association with suicidal thinking found here has a trait component, and indeed there is external evidence that suicidal behavior has this trait/state nature 
Additional studies are warranted to assess the stability of the observed association of VI
levels and suicidal ideation and to extend it to suicidal behaviors and related measures (including intent, plans and actions) through changing clinical states including euthymia as well as in chronically suicide-contemplating clients for suicide risk may lie well beyond observable mood psychopathology. Long-term actigraphic monitoring of psychomotor disturbances might be also included in psychopharmacological studies of clinical depression as an objective correlate of treatment response, as well as a measure to assist the clinical appraisal of emerging suicidal risk when psychomotor arousal precedes mood improvement 
In conclusion, the present findings indicate that VI, an objective measure derived from analysis of motility rhythms recorded by noninvasive, electronic actigraphy in depressed human subjects, correlated inversely with suicidal ideation or a wish to die, and elevated VI scores (≥3.0) distinguished unipolar from bipolar depression with favorable Bayesian characteristics. An objective biomarker of suicidal ideation could be particularly advantageous when patients are unwilling or unable to share suicidal thoughts with clinicians. A patient’s determined suicidal intent, difficulties in self-expression or an unsatisfactory therapeutic relationship are circumstances that would benefit from an objective estimate of suicidal thinking.