Conditions in which the regulation of mood itself — its depth, its height, and its stability over time — becomes the illness. Understand one axis, and the whole family falls into place.
The organizing idea
Mood runs on a spectrum — from the depths of depression, up through a stable baseline (clinicians call it euthymia), to the heights of hypomania and mania. Every mood disorder is a distinctive pattern of movement along this axis over time. Learn the axis, and the diagnoses become a set of recognisable shapes.
A person with unipolar depression only ever moves below baseline — never up into mania. That single-direction movement is the meaning of "unipolar."
A person with a bipolar condition moves in both directions — down into depression and up into hypomania or mania. "Bipolar" literally means two poles.
The distinction between these two is the most consequential judgement in all of mood psychiatry — because it completely changes the treatment.
Dips well below baseline in episodes, then returns. Never rises above.
A shallow, chronic low that never quite lifts — for years at a time.
Full mania high above baseline, often crashing into deep depression.
Smaller highs (hypomania), but the depressions run deep and disabling.
Continuous smaller swings, up and down, never reaching full episodes.
The healthy reference: normal ups and downs around a stable centre.
— — — the dashed line is euthymia: the stable baseline every pattern is measured against — — —
The building blocks
Before the diagnoses, the vocabulary. Mood disorders are assembled from a small set of defined "episodes." Which episodes a person has — and in which combination — is what determines the diagnosis.
A distinct period of persistently low mood or loss of interest/pleasure, plus enough additional symptoms to reach five, causing real distress or impairment.
Abnormally elevated, expansive, or irritable mood with markedly increased energy or activity, severe enough to cause major impairment, danger, or psychosis. Defines Bipolar I.
The same symptom picture as mania but milder and shorter — a clear, observable change in functioning, without marked impairment, hospitalisation, or psychosis. A central feature of Bipolar II.
When symptoms of the opposite pole intrude during an episode — e.g. depressive despair alongside manic agitation and racing thoughts. This state is especially distressing and carries elevated risk, so it's flagged specifically.
How the chapter is organised
In 2013 the DSM-5 split the old "mood disorders" chapter into two, placing bipolar conditions as a bridge between depression and the psychotic disorders they can resemble. The umbrella term "mood disorders" is still widely used — and still useful.
Movement only below baseline. The unifying feature is the presence of sad, empty, or irritable mood with bodily and cognitive changes that impair functioning.
Major Depressive Disorder · Persistent Depressive Disorder · Premenstrual Dysphoric Disorder · Disruptive Mood Dysregulation Disorder · induced & medical forms.
Movement in both directions. Defined by the presence — now or in the past — of at least one manic or hypomanic episode.
Bipolar I Disorder · Bipolar II Disorder · Cyclothymic Disorder · induced & medical forms.
Family one · below the line
Codes shown are DSM-5 / ICD-11.
Far more than sadness — a whole-body, whole-mind shutdown of mood, energy, thought, and pleasure.
One or more major depressive episodes: at least two weeks of low mood or anhedonia plus the SIGECAPS cluster. It touches sleep, appetite, concentration, and self-worth — not just emotion. Anhedonia, the loss of pleasure in things once enjoyed, is often the most telling sign.
A low-grade depression that becomes the background hum of life — present most days for years.
Depressed mood more days than not for at least two years (one in children), with symptoms like low energy, poor self-esteem, and hopelessness. Milder than MDD moment-to-moment, but its chronicity is its own kind of heavy.
A severe, cyclical mood disturbance tied tightly to the menstrual cycle — not "bad PMS," a distinct condition.
Marked mood swings, irritability, depression, and anxiety in the week or so before menstruation, resolving soon after it begins, month after month. The timing is the diagnostic signature; prospective daily tracking confirms it. SSRIs and hormonal approaches can both help.
A childhood diagnosis of chronic, severe irritability — created specifically to stop bipolar disorder being over-diagnosed in kids.
Persistent irritable or angry mood with frequent, severe temper outbursts out of proportion to the situation, in children aged 6–18 with onset before 10. The point of naming it was to separate these children from bipolar disorder, which was being over-applied to childhood irritability.
Depression driven by a substance, medication, or another illness — where treating the cause is the treatment.
Includes depression caused by substances or medications (e.g. some steroids, alcohol) and depression due to another medical condition (e.g. thyroid disease, stroke). This is why a good work-up checks physical health — the mood change can be the visible edge of something treatable.
Family two · both directions
Defined by at least one full manic episode — the highest reach of the mood axis.
A single manic episode is enough for the diagnosis, whether or not depression has occurred — though most people also experience major depressive episodes. Mania can include grandiosity, reduced need for sleep, racing thoughts, risk-taking, and sometimes psychosis, and may require hospitalisation.
Hypomania plus major depression — and, crucially, not a milder version of Bipolar I.
At least one hypomanic episode and at least one major depressive episode, but never a full manic episode. The highs are less extreme — but the depressive side is often the dominant, most disabling, and most dangerous feature, and people spend far more time depressed than high.
Chronic, lower-amplitude instability — persistent swings that never quite reach full episodes.
For at least two years (one in children), numerous periods of hypomanic and depressive symptoms that don't meet the threshold for full episodes, present more than half the time with no symptom-free stretch longer than two months. It can be a precursor to Bipolar I or II in some people.
The distinction that changes everything
A depressive episode can look identical whether it belongs to unipolar or bipolar illness — yet the correct treatment is different, and getting it wrong can make things worse. This is why any assessment for depression should screen carefully for past highs.
Why the screen matters
Because people seek help when they're depressed — not when they feel great during a hypomania — bipolar disorder is frequently mistaken for ordinary depression, sometimes for years. Prescribing an antidepressant on its own to someone who is actually bipolar can tip them into mania or speed up their cycling. A careful history of past elevated periods is one of the highest-value questions in psychiatry.
The fine print that guides treatment
Two people with "major depression" can be having very different experiences. Specifiers capture the texture — and often point directly to what will help.
Tension, restlessness, and dread riding alongside the mood episode.
Symptoms of the opposite pole present at the same time — higher risk, needs care.
Profound anhedonia, early waking, worse in the morning, heavy guilt.
Mood brightens with good events; oversleeping, overeating, heavy limbs.
Delusions or hallucinations, usually mood-congruent; a serious presentation.
Beginning in pregnancy or after birth; postpartum psychosis is an emergency.
Episodes recurring at a particular time of year — the basis of "SAD."
Four or more mood episodes in a year; harder to treat, needs specialist input.
Why they happen
No single cause — a convergence of biology and life. The honest model is that genes set a vulnerability, and circumstances, biology, and timing decide whether and how it surfaces.
Both run in families, but to different degrees. Bipolar disorder is among the most heritable of all psychiatric conditions (estimates around 60–85%); major depression is meaningfully but more modestly heritable (roughly 35–40%). Genes raise the odds; they don't guarantee the outcome.
The older "chemical imbalance" story (serotonin, noradrenaline, dopamine) is real but incomplete. Current thinking adds a dysregulated stress-hormone system (the HPA axis and cortisol), reduced neuroplasticity, inflammation, and disrupted circadian rhythms — the last especially central in bipolar disorder, where sleep loss can itself trigger mania.
Loss, trauma, chronic stress, and adversity are powerful triggers, particularly for a first depressive episode. Cognitive patterns — hopeless, self-critical, all-or-nothing thinking — can maintain and deepen episodes, which is exactly what therapy targets.
Getting to a diagnosis
Diagnosis is clinical — built from a careful history of mood over time, not a scan or a blood test (though bloodwork is often done to rule out thyroid problems, anaemia, and other physical contributors). The clinician maps the pattern: how low, how high, how long, how often, and how much it interferes.
Brief structured tools help track and screen. The PHQ-9 is widely used to measure depression severity and monitor response over time. The MDQ (Mood Disorder Questionnaire) screens for a history of manic or hypomanic symptoms — a practical way to catch the bipolar question that ordinary depression assessments can miss. These support clinical judgement; they don't replace it.
A note on self-recognition
Low periods and high-energy stretches are part of ordinary life. What makes a mood disorder is the intensity, duration, and degree of interference — a judgement that needs a professional who can see the whole arc. If these patterns are affecting your life, that conversation is worth having.
What genuinely helps
The encouraging headline: mood disorders are among the most treatable conditions in medicine. But depression and bipolar disorder are treated differently — which is why the unipolar-versus-bipolar question comes first.
CBT, behavioural activation, and interpersonal therapy (IPT) all have strong evidence — alone for mild-to-moderate depression, and combined with medication for more severe cases. Therapy also reduces relapse after recovery.
SSRIs and SNRIs are first-line; bupropion (helpful where fatigue and low motivation dominate) and mirtazapine are alternatives. They take a few weeks to work, and the first choice doesn't always fit — adjustment is normal, not failure.
Regular exercise has genuinely robust evidence as an adjunct. Sleep regulation, light therapy for seasonal depression, and treating any underlying medical contributor all matter.
ECT remains the most effective treatment for the most severe, psychotic, or urgent cases. Newer options include ketamine/esketamine (rapid-acting) and TMS. These are specialist, not starting points.
The cornerstone. Lithium is the long-standing gold standard; valproate and carbamazepine are alternatives; lamotrigine is especially useful for the depressive side. These stabilise the axis in both directions.
Several (e.g. quetiapine) treat acute mania and bipolar depression and are used for maintenance — sometimes alongside a mood stabiliser.
Used sparingly and generally only under the cover of a mood stabiliser, because on their own they can trigger mania or rapid cycling. This is the practical reason the diagnosis has to be right.
Psychoeducation, family-focused therapy, and interpersonal & social rhythm therapy (IPSRT) — which protects regular sleep and daily routine — measurably reduce relapse. Consistent sleep is genuinely protective.
On lithium
Decades on, lithium remains uniquely valuable: it stabilises mood in both directions and is the one psychiatric medication robustly shown to reduce suicide risk. It needs regular blood tests to stay in a safe range and to monitor the thyroid and kidneys — a manageable trade for what it offers many people.
Clearing up the common confusions
Depression is just intense sadness, or a lack of willpower — you should be able to snap out of it.
"Bipolar" means moody, or switching within minutes or hours.
Antidepressants make you numb or change who you are; taking them is a weakness.
Depression is a medical condition affecting energy, sleep, thinking, and the brain's reward system. "Try harder" is not treatment — and the inability to just snap out of it is part of the illness.
Bipolar episodes typically last days to weeks, not moments. Rapid within-day mood shifts point elsewhere (often emotional dysregulation), not classic bipolar disorder.
Medication, when it fits, lifts the fog rather than flattening the person. Finding the right one can take iteration — that's expected, not failure.
The long view
Most mood disorders are episodic and recurrent rather than constant — which cuts both ways. Episodes lift, and people return to themselves; but relapse is common, so staying on treatment through recovery (not stopping the moment things feel better) is one of the strongest protections against the next episode.
Bipolar disorder is generally a lifelong condition managed rather than cured, but with the right medication, routine, and support many people live full, stable, high-functioning lives. For depression, the outlook with treatment is genuinely good, and each recovery is real even when the illness recurs. Alongside formal treatment, the reliable helpers are consistent sleep, movement, connection with other people, and catching early warning signs before an episode gains momentum.
A word on safety, and where to turn
Mood disorders can carry thoughts of death or of not wanting to be here — this is a recognised symptom of the illness, not a character flaw, and it can ease with the right help. If you're having these thoughts, please treat it as a reason to reach out to someone today rather than something to carry alone. You don't need to be in crisis to deserve support.