Mania is a distinct period of at least one week of elevated or irritable mood, which can range from euphoria to delirium, and those experiencing hypo- or mania may exhibit three or more of the following behaviors: speak in a rapid, uninterruptible manner, short attention span, racing thoughts, increased goal-oriented activities, agitation, or they may exhibit behaviors characterized as impulsive or high-risk, such as hypersexuality or excessive spending. To meet the definition for a manic episode, these behaviors must impair the individual's ability to socialize or work. If untreated, a manic episode usually lasts three to six months.

People with hypomania or mania may experience a decreased need of sleep, impaired judgment, and speak excessively and very rapidly. Manic individuals often have a history of substance abuse developed over years as a form of "self-medication". At the more extreme, a person in a full blown manic state can experience psychosis; a break with reality, a state in which thinking is affected along with mood. They may feel unstoppable, or as if they have been "chosen" and are on a "special mission", or have other grandiose or delusional ideas. This may lead to violent behavior and, sometimes, hospitalization in an inpatient psychiatric hospital. The severity of manic symptoms can be measured by rating scales such as the Young Mania Rating Scale, though questions remain about the reliability of these scales.

The onset of a manic or depressive episode is often foreshadowed by sleep disturbances. Mood changes, psychomotor and appetite changes, and an increase in anxiety can also occur up to three weeks before a manic episode develops.

Hypomania is the milder form of mania, defined as at least four days of the same criteria as mania, but does not cause a significant decrease in the individual's ability to socialize or work, lacks psychotic features such as delusions or hallucinations, and does not require psychiatric hospitalization. Overall functioning may actually increase during episodes of hypomania and is thought to serve as a defense mechanism against depression by some. Hypomanic episodes rarely progress to full blown manic episodes. Some people who experience hypomania show increased creativity while others are irritable or demonstrate poor judgment.

Hypomania may feel good to some persons who experience it, though most people who experience hypomania state that the stress of the experience is very painful. Bipolar people who experience hypomania, however, tend to forget the effects of their actions on those around them. Even when family and friends recognize mood swings, the individual will often deny that anything is wrong. What might be called a "hypomanic event", if not accompanied by depressive episodes, is often not deemed problematic, unless the mood changes are uncontrollable, volatile, or mercurial. Most commonly, symptoms continue for a few weeks to a few months.

The essential feature of bipolar I disorder is a clinical course characterized by the occurrence of one or more manic episodes or mixed episodes (DSM-IV-TR, 2000). Often, individuals have had one or more major depressive episodes. One episode of mania is sufficient to make the diagnosis of bipolar disorder; the patient may or may not have history of major depressive disorder. Episodes of substance-induced mood disorder due to the direct effects of a medication, or other somatic treatments for depression, drug abuse, or toxin exposure, or of mood disorder due to a general medical condition need to be excluded before a diagnosis of bipolar I disorder can be made. In addition, the episodes must not be better accounted for by schizoaffective disorder or superimposed on schizophrenia, schizophreniform disorder, delusional disorder, or a psychotic disorder not otherwise specified.

Cyclothymia is characterized by short cycles of baseline, stable periods of not over two months and numerous swings between depression and hypomania that fail to meet the severity of sustained duration criterion for major affective syndromes for at least two years.

Depressive/dysthymic episodes. Symptoms of the "depressive/dysthymic phase" may include any of the following conditions:difficulty making decisions, problems concentrating, poor memory recall, guilt, self-criticism, low self-esteem, pessimism, self-destructive thinking, constant sadness, apathy, hopelessness, helplessness and irritability. Also common are quick temper, poor judgment, lack of motivation, social withdrawal, appetite change, lack of sexual desire, self-neglect, fatigue, insomnia and sleepiness.

Hypomanic episodes. Symptoms of the "hypomanic episode" may include any of the following conditions: unusually good mood or cheerfulness (euphoria), extreme optimism, inflated self-esteem, rapid speech, racing thoughts, aggressive or hostile behavior, lack of consideration for others, agitation, massively increased physical activity, risky behavior, spending sprees, increased drive to perform or achieve goals, increased sexual drive, decreased need for sleep, tendency to be easily distracted, and inability to concentrate.

Depressive mixed states occur when patients experience depression and non-euphoric, usually subsyndromal, hypomania at the same time. As mentioned previously, it is particularly difficult to diagnose BP-II when a patient is in this state.

In a mixed state, mood is depressed, but the following symptoms of hypomania present as well:

- Irritability

- Mental overactivity

- Behavioral overactivity

Mixed states are associated with greater levels of suicidality than non-mixed depression. Antidepressants may increase this risk.

Hypomania is the signature characteristic of Bipolar II disorder. It is a state characterized by euphoria and/or an irritable mood. In order for an episode to qualify as hypomanic, the individual must also present three or more of the below symptoms, and last at least four consecutive days and be present most of the day, nearly every day

- Inflated self-esteem or grandiosity.

- Decreased need for sleep (e.g., feels rested after only 3 hours of sleep).

- More talkative than usual or pressure to keep talking.

- Flight of ideas or subjective experience that thoughts are racing.

- Distractibility (i.e., attention too easily drawn to unimportant or irrelevant external stimuli), as reported or observed.

- Increase in goal-directed activity (either socially, at work or school, or sexually) or psychomotor agitation.

- Excessive involvement in activities that have a high potential for painful consequences (e.g., engaging in unrestrained buying sprees, sexual indiscretions, or foolish business investments).

It is important to distinguish between hypomania and mania. Mania is generally greater in severity and impairs function, sometimes leading to hospitalization and in the most severe cases, psychosis. In contrast, hypomania usually increases functioning. For this reason, it is not uncommon for hypomania to go unnoticed. Often it is not until individuals are in a depressive episode that they seek treatment, and even then their history of hypomania may go undiagnosed. Even though hypomania may increase functioning, episodes need to be treated because they may precipitate a depressive episode.

Schizoaffective disorder is defined by "mood disorder-free psychosis" in the context of a long-term psychotic and mood disorder. Psychosis must meet criterion A for schizophrenia which may include delusions, hallucinations, disorganized speech, thinking or behavior and negative symptoms. Both delusions and hallucinations are classic symptoms of psychosis. Delusions are false beliefs which are strongly held despite evidence to the contrary. Beliefs should not be considered delusional if they are in keeping with cultural beliefs. Delusional beliefs may or may not reflect mood symptoms (for example, someone experiencing depression may or may not experience delusions of guilt). Hallucinations are disturbances in perception involving any of the five senses, although auditory hallucinations (or "hearing voices") are the most common. A lack of responsiveness or negative symptoms include alogia (lack of spontaneous speech), blunted affect (reduced intensity of outward emotional expression), avolition (loss of motivation), and anhedonia (inability to experience pleasure). Negative symptoms can be more lasting and more debilitating than positive symptoms of psychosis.

Mood symptoms are of mania, hypomania, mixed episode, or depression, and tend to be episodic rather than continuous. A mixed episode represents a combination of symptoms of mania and depression at the same time. Symptoms of mania include elevated or irritable mood, grandiosity (inflated self-esteem), agitation, risk-taking behavior, decreased need for sleep, poor concentration, rapid speech, and racing thoughts. Symptoms of depression include low mood, apathy, changes in appetite or weight, disturbances in sleep, changes in motor activity, fatigue, guilt or feelings of worthlessness, and suicidal thinking.

A single manic episode, in the absence of secondary causes, (i.e., substance use disorder, pharmacologic, general medical condition) is sufficient to diagnose bipolar I disorder. Hypomania may be indicative of bipolar II disorder. Manic episodes are often complicated by delusions and/or hallucinations; should the psychotic features persist for a duration significantly longer than the episode of mania (two weeks or more), a diagnosis of schizoaffective disorder is more appropriate. Certain of "obsessive-compulsive spectrum" disorders as well as impulse control disorders share the name "mania," namely, kleptomania, pyromania, and trichotillomania. Despite the unfortunate association implied by the name, however, no connection exists between mania or bipolar disorder and these disorders.

B deficiency can also cause characteristics of mania and psychosis.

Hyperthyroidism can produce similar symptoms to those of mania, such as agitation, elevated mood, increased energy, hyperactivity, sleep disturbances and sometimes, especially in severe cases, psychosis.

Hypomania is a lowered state of mania that does little to impair function or decrease quality of life. It may, in fact, increase productivity and creativity. In hypomania, there is less need for sleep and both goal-motivated behaviour and metabolism increase. Though the elevated mood and energy level typical of hypomania could be seen as a benefit, mania itself generally has many undesirable consequences including suicidal tendencies, and hypomania can, if the prominent mood is irritable rather than euphoric, be a rather unpleasant experience. By definition, hypomania cannot feature psychosis, nor can it require psychiatric hospitalisation (voluntary or involuntary).

"Mood disorder due to a general medical condition" is used to describe manic or depressive episodes which occur secondary to a medical condition. There are many medical conditions that can trigger mood episodes, including neurological disorders (e.g. dementias), metabolic disorders (e.g. electrolyte disturbances), gastrointestinal diseases (e.g. cirrhosis), endocrine disease (e.g. thyroid abnormalities), cardiovascular disease (e.g. heart attack), pulmonary disease (e.g. chronic obstructive pulmonary disease), cancer, and autoimmune diseases (e.g. rheumatoid arthritis).

Cyclothymia (), also called cyclothymic disorder, is a type of chronic mood disorder widely considered to be a more chronic but milder or subthreshold form of bipolar disorder. Cyclothymia is characterized by numerous mood swings, with periods of hypomanic symptoms that do not meet criteria for a manic episode, alternating with periods of mild or moderate symptoms of depression that do not meet criteria for a major depressive episode.

An individual with cyclothymia may feel stable at a baseline level but experience a noticeable shift to an emotional high during subthreshold hypomanic episodes of elation or euphoria, with symptoms similar to those of mania but less severe, and often cycle to emotional lows with moderate depressive symptoms. To meet the diagnostic criteria for cyclothymia, a person must experience this alternating pattern of emotional highs and lows for a period of at least two years with no more than two consecutive symptom-free months. For children and adolescents, the duration must be at least one year.

The diagnosis of cyclothymia is rare compared to other mood disorders. Diagnosis of cyclothymia entails the absence of any major depressive episode, manic episode or mixed episode, which would qualify the individual for diagnosis of other mood disorders. When a major episode manifests after an initial diagnosis of cyclothymia, the individual may qualify for a diagnosis of bipolar I or bipolar II disorder. Although estimates vary greatly, 15–50% of cases of cyclothymia later advance to the diagnostic criteria for bipolar I and/or bipolar II disorder (with cyclothymic features). Although the emotional highs and lows of cyclothymia are less extreme than those of bipolar disorder and generally do not cause the same conditions, the symptomatology, longitudinal course, family history and treatment response of cyclothymia are consistent with bipolar spectrum.

Lifetime prevalence of cyclothymic disorder is 0.4–1%. Frequency appears similar in men and women, though women more often seek treatment. People with cyclothymia during periodic hypomania (euphoria) tend to feel an inflated self-worth, self-confidence and elation, often with rapid speech, racing thoughts, not much need to sleep, increased aggression and impulsive behavior, showing little regard for consequences of decisions—but may sometimes be somewhat, fully or hyper-productive for a period of several days at a time.

Bipolar I disorder (BD-I; pronounced "type one bipolar disorder") is a bipolar spectrum disorder characterized by the occurrence of at least one manic or mixed episode. Most patients also, at other times, have one or more depressive episodes, and all experience a hypomanic stage before progressing to full mania.

It is a type of bipolar disorder, and conforms to the classic concept of manic-depressive illness, which can include psychosis during mood episodes. The difference with bipolar II disorder is that the latter requires that the individual must never have experienced a full manic or mixed-manic episode - only less severe hypomanic episode(s).

A mood disorder can be classified as substance-induced if its etiology can be traced to the direct physiologic effects of a psychoactive drug or other chemical substance, or if the development of the mood disorder occurred contemporaneously with substance intoxication or withdrawal. Also, an individual may have a mood disorder coexisting with a substance abuse disorder. Substance-induced mood disorders can have features of a manic, hypomanic, mixed, or depressive episode. Most substances can induce a variety of mood disorders. For example, stimulants such as amphetamine, methamphetamine, and cocaine can cause manic, hypomanic, mixed, and depressive episodes.

Melancholic depression, or depression with melancholic features, is a DSM-IV subtype of clinical depression requiring at least one of the following symptoms:

- Anhedonia (the inability to find pleasure in positive things)

- Lack of mood reactivity (i.e. mood does not improve in response to positive events)

And at least three of the following:

- Depression that is subjectively different from grief or loss

- Severe weight loss or loss of appetite

- Psychomotor agitation or retardation

- Early morning awakening

- Guilt that is excessive

- Worse mood in the morning

Melancholic features apply to an episode of depression that occurs as part of either major depressive disorder or bipolar disorder I or II.

Melancholic depression is often considered to be a biologically based and particularly severe form of depression. Treatment involves antidepressants, electroconvulsive therapy, or other empirically supported treatments such as cognitive behavioral therapy and interpersonal therapy for depression. A 2008 analysis of a large study of patients with unipolar major depression found a rate of 23.5% for melancholic features. It was the first form of depression extensively studied, and many of the early symptom checklists for depression reflect this.

The incidence of melancholic depression has been found to increase when the temperature and/or sunlight are low.

According to the DSM-IV, the "melancholic features" specifier may be applied to the following only:

1. Major depressive episode, single episode

2. Major depressive episode, recurrent episode

3. Bipolar I disorder, most recent episode depressed

4. Bipolar II disorder, most recent episode depressed

As affirmed by the "Diagnostic and Statistical Manual of Mental Disorders", fifth edition (DSM-V), the symptomology specifier "with mixed features" can be applied to manic episodes of bipolar I disorder, hypomanic episodes of either bipolar I disorder or bipolar II disorder, and depressive episodes of either bipolar disorder or major depressive disorder, with at least three concurrent features of the opposite polarity being present. As a result, the presence of "mixed features" are now recognized in patients with bipolar II disorder and major depression; as earlier noted, however, although it is customary to withhold a diagnosis of a bipolar disorder until a manic or hypomanic episode appears, the presence of such features in a depressed patient even with no history of discrete mania or hypomania is strongly suggestive of the disorder.

Two features of both mania or hypomania and depression may superficially overlap and even resemble each other, namely "an increase in goal-directed activity" (psychomotor acceleration) vs. psychomotor agitation and "flight of ideas" and "racing thoughts" vs. depressive rumination. Attending to the patient's experiences is very important. In the psychomotor agitation commonly seen in depression, the "nervous energy" is always overshadowed by a strong sense of exhaustion and manifests as purposeless movements (e.g., pacing, hand-wringing); in psychomotor acceleration, however, the excess in movement stems from an abundance of energy and is often channelled and purposeful. Likewise, in depressive rumination, the patient experiences the repetitive thoughts as heavy, leaden, and plodding; in psychic acceleration, however, (as seen in mania or hypomania) the thoughts move in a rapid progression, with many themes, rather than a singular one, being touched upon. Even when such experiences are accounted for on the basis of depression, the possibility does still exist, however, that the depressive episode may be complicated by other manic or hypomanic symptoms, in which case it is often prudent to attend to the patient's personal and family history (e.g., family history of bipolar disorder, early age of onset) to determine whether or not the patient has bipolar disorder.

Traditionally, a mixed affective state, formerly known as a mixed-manic or mixed episode, has been defined as a state wherein features unique to both depression and mania—such as despair, fatigue, morbid or suicidal ideation; racing thoughts, pressure of activity, and heightened irritability—occur either simultaneously or in very short succession.

Previously, the diagnostic criteria for both a manic and depressive episode had to be met in a consistent and sustained fashion, with symptoms enduring for at least a week (or any duration if psychiatric hospitalization was required), thereby restricting the official acknowledgement of mixed affective states to only a minority of patients with bipolar I disorder. In current (DSM-V) nomenclature, however, a "mixed episode" no longer stands as an episode of illness unto itself; rather, the symptomology specifier "with mixed features" can be applied to any major affective episode (manic, hypomanic, or depressive), meaning that they are now officially recognized in patients with, in addition to bipolar I disorder, bipolar II disorder and, by convention, major depressive disorder; a depressive mixed state in a patient, however, even in the absence of discrete periods of mania or hypomania, effectively rules out unipolar depression. Although convention dictates diagnosing such patients with major depressive disorder, it is more appropriate to classify such a patient as having an "Other Specified Bipolar or Related Disorder" and certainly following up such patients in the long-term, to ascertain the eventual appearance of an episode of mania or hypomania (with or without mixed features), which would prompt diagnostic revision. In those uncommon cases wherein the diagnostic criteria for both a manic and depressive episode are met, constituting what were heretofore classified as "mixed episodes", such a patient would be diagnosed as having a "manic episode with mixed features", because of the substantial social and occupational dysfunction inherent in such a state.

Atypical depression, or depression with atypical features as it has been known in the DSM IV, is depression that shares many of the typical symptoms of the psychiatric syndromes major depression or dysthymia but is characterized by improved mood in response to positive events. In contrast, people with melancholic depression generally do not experience an improved mood in response to normally pleasurable events. Atypical depression also features significant weight gain or an increased appetite, hypersomnia, a heavy sensation in the limbs and interpersonal rejection sensitivity that results in significant social or occupational impairment.

Despite its name, "atypical" depression does not mean it is uncommon or unusual. The reason for its name is twofold: (1) it was identified with its "unique" symptoms subsequent to the identification of melancholic depression and (2) its responses to the two different classes of antidepressants that were available at the time were different from melancholic depression (i.e., MAOIs had clinically significant benefits for atypical depression, while tricyclics did not).

Atypical depression is two to three times more common in women than in men. Individuals with atypical features tend to report an earlier age of onset (e.g. while in high school) of their depressive episodes, which also tend to be more chronic and only have partial remission between episodes. Younger individuals may be more likely to have atypical features, whereas older individuals may more often have episodes with melancholic features. Atypical depression has high comorbidity of anxiety disorders, carries more risk of suicidal behavior, and has distinct personality psychopathology and biological traits.

Atypical depression is more common in individuals with bipolar I, bipolar II, cyclothymia and seasonal affective disorder. Depressive episodes in bipolar disorder tend to have atypical features, as does depression with seasonal patterns.

Schizoaffective disorder (SZA, SZD or SAD) is a mental disorder characterized by abnormal thought processes and deregulated emotions. The diagnosis is made when the person has features of both schizophrenia and a mood disorder—either bipolar disorder or depression—but does not strictly meet diagnostic criteria for either alone. The bipolar type is distinguished by symptoms of mania, hypomania, or mixed episode; the depressive type by symptoms of depression only. Common symptoms of the disorder include hallucinations, paranoid delusions, and disorganized speech and thinking. The onset of symptoms usually begins in young adulthood, currently with an uncertain lifetime prevalence because the disorder was redefined, but DSM-IV prevalence estimates were less than 1 percent of the population, in the range of 0.5 to 0.8 percent. Diagnosis is based on observed behavior and the person's reported experiences.

Genetics, neurobiology, early and current environment, behavioral, social, and experiential components appear to be important contributory factors; some recreational and prescription drugs may cause or worsen symptoms. No single isolated organic cause has been found, but extensive evidence exists for abnormalities in the metabolism of tetrahydrobiopterin (BH4), dopamine, and glutamic acid in people with schizophrenia, psychotic mood disorders, and schizoaffective disorder. People with schizoaffective disorder are likely to have co-occurring conditions, including anxiety disorders and substance use disorder. Social problems such as long-term unemployment, poverty and homelessness are common. The average life expectancy of people with the disorder is shorter than those without it, due to increased physical health problems from an absence of health promoting behaviors such as a sedentary lifestyle, and a higher suicide rate.

The mainstay of current treatment is antipsychotic medication combined with mood stabilizer medication or antidepressant medication, or both. There is growing concern by some researchers that antidepressants may increase psychosis, mania, and long-term mood episode cycling in the disorder. When there is risk to self or others, usually early in treatment, hospitalization may be necessary. Psychiatric rehabilitation, psychotherapy, and vocational rehabilitation are very important for recovery of higher psychosocial function. As a group, people with schizoaffective disorder diagnosed using DSM-IV and criteria have a better outcome than people with schizophrenia, but have variable individual psychosocial functional outcomes compared to people with mood disorders, from worse to the same. Outcomes for people with DSM-5 diagnosed schizoaffective disorder depend on data from prospective cohort studies, which haven't been completed yet.

In DSM-5 and ICD-10, schizoaffective disorder is in the same diagnostic class as schizophrenia, but not in the same class as mood disorders. The diagnosis was introduced in 1933, and its definition was slightly changed in the DSM-5, published in May 2013, because the DSM-IV schizoaffective disorder definition leads to excessive misdiagnosis. The changes made to the schizoaffective disorder definition were intended to make the DSM-5 diagnosis more consistent (or reliable), and to substantially reduce the use of the diagnosis. Additionally, the DSM-5 schizoaffective disorder diagnosis can no longer be used for first episode psychosis.

Major depression significantly affects a person's family and personal relationships, work or school life, sleeping and eating habits, and general health. Its impact on functioning and well-being has been compared to that of other chronic medical conditions such as diabetes.

A person having a major depressive episode usually exhibits a very low mood, which pervades all aspects of life, and an inability to experience pleasure in activities that were formerly enjoyed. Depressed people may be preoccupied with, or ruminate over, thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness, hopelessness, and self-hatred. In severe cases, depressed people may have symptoms of psychosis. These symptoms include delusions or, less commonly, hallucinations, usually unpleasant. Other symptoms of depression include poor concentration and memory (especially in those with melancholic or psychotic features), withdrawal from social situations and activities, reduced sex drive, irritability, and thoughts of death or suicide. Insomnia is common among the depressed. In the typical pattern, a person wakes very early and cannot get back to sleep. Hypersomnia, or oversleeping, can also happen. Some antidepressants may also cause insomnia due to their stimulating effect.

A depressed person may report multiple physical symptoms such as fatigue, headaches, or digestive problems; physical complaints are the most common presenting problem in developing countries, according to the World Health Organization's criteria for depression. Appetite often decreases, with resulting weight loss, although increased appetite and weight gain occasionally occur. Family and friends may notice that the person's behavior is either agitated or lethargic. Older depressed people may have cognitive symptoms of recent onset, such as forgetfulness, and a more noticeable slowing of movements. Depression often coexists with physical disorders common among the elderly, such as stroke, other cardiovascular diseases, Parkinson's disease, and chronic obstructive pulmonary disease.

Depressed children may often display an irritable mood rather than a depressed mood, and show varying symptoms depending on age and situation. Most lose interest in school and show a decline in academic performance. They may be described as clingy, demanding, dependent, or insecure. Diagnosis may be delayed or missed when symptoms are interpreted as normal moodiness.

The DSM-IV-TR defines Atypical Depression as a subtype of Major Depressive Disorder with Atypical Features, characterized by:

1. Mood reactivity (i.e., mood brightens in response to actual or potential positive events)

2. At least two of the following:

- Significant weight gain or increase in appetite;

- Hypersomnia (sleeping too much, as opposed to the insomnia present in melancholic depression);

- Leaden paralysis (i.e., heavy, leaden feelings in arms or legs);

- Long-standing pattern of interpersonal rejection sensitivity (not limited to episodes of mood disturbance) that results in significant social or occupational impairment.

3. Criteria are not met for With Melancholic Features or With Catatonic Features during the same episode.

Recurrent brief depression (RBD) defines a mental disorder characterized by intermittent depressive episodes, not related to menstrual cycles in women, occurring between approximately 6-12 times per year, over at least one year or more fulfilling the diagnostic criteria for major depressive episodes (DSM-IV and ICD-10) except for duration which in RBD is less than 14 days, typically 2–4 days. Despite the short duration of the depressive episodes, such episodes are severe and suicidal ideation and impaired function is rather common. The majority of patients with RBD also report symptoms of anxiety and increased irritability. Hypersomnia is also rather frequent. About 1/2 of patients fulfilling diagnostic criteria for RBD may have additional short episodes of brief hypomania which is a severity marker of RBD. RBD may be the only mental disorder present, but RBD may also occur as part of a history of recurrent major depressive episodes or bipolar disorders. RBD is also seen among some patients with personality disorders.

Nearly every day, the person may sleep excessively, known as hypersomnia, or not enough, known as insomnia.

Insomnia is the most common type of sleep disturbance for people who are clinically depressed and is often associated with a melancholic type of depression. Symptoms of insomnia include trouble falling asleep, trouble staying asleep, and/or waking up too early in the morning.

Hypersomnia is a less common type of sleep disturbance. It may include sleeping for prolonged periods at night or increased sleeping during the daytime. The sleep may not be restful, and the person may feel sluggish despite many hours of sleep. This impacts their everyday activities and ability to focus at home or work. According to the United States National Library of Medicine, people with seasonal affective disorder (SAD) may sleep longer during the winter months. Hypersomnia is often associated with an atypical depression. Hypersomnia is not as common as insomnia and up to 40% of people exhibit hypersomnia from time to time.

A person experiencing a depressive episode may have a marked loss or gain of weight (such as 5% of their body weight in one month) or a change in appetite.

Changes in appetite take on two manifestations: under- or over-eating.

In the first instance, some people never feel hungry, can go long periods without wanting to eat, or may forget to eat; if they do eat, a small amount of food may be sufficient. In children, failure to make expected weight gains may be counted towards this criteria. Under-eating is often associated with a melancholic type of depression.

In the second instance, some people tend toward an increase in appetite and may gain significant amounts of weight. They may crave certain types of food, such as sweets or carbohydrates. People with seasonal affective disorder (SAD) often crave foods high in carbohydrates. Over-eating is often associated with a type of depression called atypical depression.

To meet criteria for a major depressive episode, a patient must have 5 of these 9 symptoms nearly every day for at least 2 weeks.

1. Depressed or sad mood

2. Anhedonia (loss of interest in pleasurable activities)

3. Sleep disturbance (increased or decreased sleep)

4. Appetite disturbance (increased or decreased appetite) typically with weight change

5. Energy disturbance (increased or decreased energy/activity level), usually fatigue

6. Poor memory and/or concentration

7. Feelings of guilt or worthlessness

8. Psychomotor retardation or agitation (a change in mental and physical speed perceived by other people)

9. Thoughts of wishing you were dead; suicidal ideation or suicide attempts

The lifetime prevalence of RBD has been estimated at 2.6 to 10.0%, and the one-year prevalence at 5.0-8.2%. The World Health Organization project on "Psychological problems in general health care", which was based on primary care samples, reported a one-year prevalence of 3.7 – 9.9%. However none of these studies differentiate between RBD with and without a history of other mood disorders (e.g. major depression). DSM-IV field trial estimated the life-time of RBD only to be about 2%.