Comorbidity between hypersomnia disorders and mood symptoms, particularly depression, is frequently reported in both clinical and research settings. However, the causal relationship between the two conditions remains unclear. Hypersomnia symptoms may also be misdiagnosed as depression, because MDD symptoms according to the DSM-IV-TR are common features of hypersomnia disorders.
Narcolepsy with cataplexy
Narcolepsy is an orphan sleep disorder (0.026% of the general population) characterized by a clinical history of EDS and abnormal manifestations of dissociated REM sleep, such as cataplexy (that is, sudden loss of muscle tone triggered by strong emotions), hypnagogic hallucinations, and sleep paralysis [
59]. Narcolepsy typically starts during adolescence, a critical period of normal development and interpersonal relationship building [
60]. Cataplexy is the best clinical diagnostic marker for the disease, occurring in 70% to 80% of patients. Two separate entities are individualized: narcolepsy with and without cataplexy [
11]. Narcolepsy diagnosis requires nocturnal PSG recording followed by the MSLT, the latter showing a mean sleep latency <8 minutes and two or more sleep onset REM periods (SOREMPs).
For 120 years after it was identified, narcolepsy was attributed to psychiatric etiologies [
61]. In the 2000s, however, it was determined that when cataplexy is present, narcolepsy is almost always caused by an immune-mediated destruction of orexin/hypocretin neurons located in the lateral hypothalamus [
62,
63]. Accordingly, low-undetectable cerebrospinal hypocretin-1 is observed in almost all cases of narcolepsy with cataplexy, but in only 10% to 20% in narcolepsy without cataplexy [
64‐
67]. Studies linking loss of hypocretin neurons to human narcolepsy have spurred interest in improving the disease phenotype. Neuroanatomical studies indicate that hypocretin projections are widespread, including the cortex, the basal forebrain, limbic structures (for example, amygdala, ventral tegmental area), the thalamus, most of the brainstem (that is, locus coeruleus, raphe nucleus, and cholinergic tegmental nuclei) and the spinal cord [
68]. The hypocretins act primarily as excitatory neurotransmitters to control monoaminergic and cholinergic neuron activity. Hypocretin deficiency induces a cholinergic–monoaminergic imbalance, with primary effects on vigilance as well as other functions, including mood regulation. Because hypocretins are also involved in neuroendocrine functions and stress reactions through stimulation of the hypothalamus–pituitary–adrenal axis, hypocretin deficiency
per se may trigger mood disturbances and psychological alterations through diverse pathways.
High levels of psychopathology were frequently reported in cross-sectional studies in narcolepsy, with a high prevalence of self-reported depressive symptoms [
69‐
72]. In a lifetime approach case–control study using DSM-IV-TR criteria, increased psychotic symptoms were found in narcolepsy with cataplexy, with no increase in depression frequency [
73]. However, psychotic-like symptoms were induced by amphetamine intake and not by the disease
per se, and symptoms were resolved when the dose was lowered or treatment changed to modafinil [
73]. Another case–control study found mood disorder symptoms in one-third of narcolepsy patients. Nevertheless, no significant difference was found between patients and controls regarding the formal mood disorder diagnosis [
74]. In contrast, more than half the patients had anxiety or panic attacks and 35% had an anxiety disorder [
74]. Regarding depressive symptomatology, the frequency of moderate to severe symptoms ranged from 15% to 37% in narcolepsy [
75‐
78]. Our large cross-sectional narcolepsy study found that depressive symptoms (using the BDI-II) were associated with greater EDS severity, greater alterations in physical and mental health quality of life, as well as the presence of REM sleep manifestations, such as cataplexy, hypnagogic hallucinations and sleep paralysis [
76]. Although depressed patients generally tend to overscore on any scale, these data reinforced reported associations between depressive symptoms, hypnagogic hallucinations, and sleep paralysis in a population-based study [
79]. A similar frequency of depressive symptoms was found in other narcoleptic populations and using other questionnaires [
80‐
82]. Interestingly, in a five-year cohort study of patients with narcolepsy-cataplexy, mood symptoms remained relatively stable, with 25% of patients showing constant moderate to severe mood symptoms across assessments [
81]. Globally, a high frequency of depressive symptoms has been systematically reported in narcolepsy with cataplexy, supporting the hypothesis of an endogenous depression [
61,
76,
81]. This condition was previously suggested based on reduced nighttime REM sleep latency, increased REM sleep pressure and sleep fragmentation in both narcolepsy and major depression.
Although there is no cure for narcolepsy, psychostimulants such as modafinil, methylphenidate, amphetamine, and sodium oxybate are used to treat EDS and sleep attacks. Cataplexy is managed with sodium oxybate and antidepressant drugs [
59]. Hence, most drugs used to manage cataplexy have mood-modifying properties. When assessing treatment effects on disease severity and psychological consequences, we observed that patients with narcolepsy with cataplexy treated with anticataplectics (mainly tricyclic agents and selective serotonin reuptake inhibitors) had more depressive symptoms and greater quality of life alterations compared to patients treated with stimulants alone [
76]. Higher baseline severity of REM sleep manifestations would normally lead to higher prescriptions of anticataplectics. However, our study revealed that anticataplectics at doses prescribed for cataplexy management were ineffective in treating depressive symptoms [
76].
Recent studies also support the role of specific hypocretin receptors in the modulation of depression-like behavior. A behavioral mice study after genetic or pharmacologic inhibition of hypocretin receptor signaling suggested that the hypocretin activity balance at either receptor 1 or receptor 2 produced an anti-depressant- or pro-depressant-like effect depending on the subtype activated [
83].
Outside of the narcolepsy scope, some human studies have compared the levels of hypocretin-1 in the cerebrospinal fluid (CSF) in patients with mood disorders with the levels in healthy controls leading to controversial results. A reduced amplitude in diurnal variations of hypocretin-1 has been found in patients with bipolar or unipolar depression [
84]. Brundin and co-workers found that suicidal patients with major depressive disorder had significantly lower CSF hypocretin levels than other suicidal patients [
85]. In contrast other studies found similar CSF hypocretin-1 levels in patients with MDD and controls [
86,
87]. In addition, CSF hypocretin-1 levels did not correlate with the severity of depressive episode, the symptoms of depression or the number of episodes [
86,
87]. To our best knowledge, no studies have reported whether CSF hypocretin-1 levels are altered in depressive patients with hypersomnia or not. However, some recent results hold promise for the use of non-selective hypocretin-1 and −2 agonists and antagonists to treat several neuropsychiatric disorders, including narcolepsy, insomnia, and drug addiction. Nevertheless, potential unwanted side effects must be carefully monitored.
Idiopathic hypersomnia
The prevalence of idiopathic hypersomnia (IH) in the general population is unknown. The age of symptom onset varies, but is frequently between 10 and 30 years [
88,
89]. Its pathophysiology is almost totally unknown, with no clear biological or genetic markers. Two forms of IH have been individualized: IH with and without long sleep time (LST) [
11]. IH with LST is characterized by three major symptoms: (1) constant daily excessive sleepiness with unwanted and prolonged (≥1 hour) naps, less irresistible than in narcolepsy, and unrefreshing irrespective of duration; (2) long nocturnal sleep, uninterrupted and prolonged, with ≥10 hours of sleep; and (3) difficulty awakening after nighttime or daytime sleep, with frequent problems reacting adequately to external stimuli upon awakening, called ‘sleep drunkenness’ or ‘sleep inertia.’ IH without LST is characterized by isolated EDS. Daytime sleep episodes may be more irresistible, shorter, and more refreshing than in IH with LST. Nocturnal sleep in IH without LST is normal, with rare sleep inertia. However, some patients share clinical symptoms of both forms of IH, with prolonged nocturnal sleep time without sleep inertia, and others with nocturnal sleep of normal duration but with major sleep drunkenness [
25,
88,
89]. Other symptoms such as mood changes (but not major depression), headache, and manifestations of neurovegetative impairment may be reported. IH diagnosis is based on clinical features and PSG followed by the MSLT (for IH without LST) or 24-hour continuous PSG under an
ad libitum sleep/wake protocol (for IH with LST) to objectively confirm hypersomnia, ascertain the diagnosis and rule out other causes of hypersomnia (for example, insufficient sleep, sleep apnea syndrome, narcolepsy).
Depressive symptoms were noted in 15% to 25% using clinic-based samples of patients affected with IH, values being always higher than in the general population [
55,
56,
76,
90]. Mood changes not qualifying for a mood disorder diagnosis may precede or follow EDS onset and evolve independently. However, MDD (being an exclusion criterion) is incompatible with an IH diagnosis, unlike narcolepsy-cataplexy, which is potentially associated with mood disorders. Hence, it is unrealistic to estimate the frequency of mood disorders and depressive symptom severity in both forms of IH. However, we may suggest a bridge between hypersomnia associated with mood disorders or with depressive symptoms only and IH, with difficulty distinguishing the two conditions in some cases. Prospective studies are needed to unravel this issue, more specifically, to clarify whether mood changes in IH are consequent to difficulty adapting to the disease or whether they indicate a primary brain dysfunction.