Bipolar Disorder in the Brain

Bipolar disorder is a mood condition defined by episodes of mania or hypomania, usually alongside episodes of depression, in which energy, sleep need, judgement and speed of thought all move together.

Also called: Manic depression, Bipolar affective disorder, Bipolar I disorder, Bipolar II disorder.

ICD-10 F31 · ICD-11 6A60

Strongly inherited. Set off by sleep and drive.

The picture most people have is a chemical that swings: too much of it in mania, too little in depression.

The research shows something different. Bipolar disorder is one of the most strongly inherited conditions in psychiatry. The brain differences are real, but they are small. And two very ordinary things often set off the episodes. One is lost sleep. The other is the drive that comes with chasing a goal.

Here are five findings. They run from the least settled to the most useful.

Too much dopamine up, too little down

Evidence: contested

The explanation you will hear most. Dopamine is involved. A simple swing is not what the scans show.

This story comes from drugs. Stimulants raise dopamine, and they can cause a state that looks like mania. The medicines that calm mania fastest block dopamine. So the reasoning goes like this. Mania is too much dopamine, and depression is too little.

Researchers then looked directly, and the picture was less tidy. A 2017 review of brain scans did find some changes in the dopamine system. But the results differed from study to study, and many of them were small.

The reasoning also has a gap in it. A drug that calms mania by blocking dopamine shows that dopamine is part of the circuit. It does not show that the dopamine level was the fault.

Dopamine still matters here. The honest version is that it is one part of a system controlling energy and reward. It is not a single dial that swings up and down.

Brakes and a hill

Brakes stop a car rolling down a hill. That does not mean the hill was caused by missing brakes.

Why the myth is sticky

It is simple, it makes the highs and lows sound like two ends of one thing, and it matches how medicines are described. None of that makes it the cause.

Strongly inherited, and shared

Evidence: established

This is one of the most inherited conditions there is. Its genes also overlap with other conditions.

Twin and family studies put the heritability of bipolar disorder at around 60 to 80%. That means genes explain most of the difference in risk between one person and another.

There is no single bipolar gene. A 2025 study looked at more than 150,000 people with bipolar disorder. It found 298 places in the genome linked to it. Each one adds a very small amount of risk.

Many of those genetic risks are shared with other conditions. Bipolar I overlaps most with schizophrenia. Bipolar II overlaps more with depression. The diagnoses are separate. The biology underneath them is not neatly divided.

Inherited does not mean certain. Most children of a parent with bipolar disorder never develop it. Genes set the level of risk. Life events, sleep and stress affect whether episodes actually happen.

Three fields side by side

The fences between them are real. The soil runs underneath all three. What grows in one field often grows at the edge of the next one.

A slightly thinner cortex, and lithium

Evidence: established

The differences are small on average. One medicine may protect against them.

The cortex is the folded outer layer of the brain. The largest scan study of bipolar disorder included more than 6,500 people. It found the cortex was slightly thinner in people with bipolar disorder, mainly at the front and the sides.

The same study found something nobody expected. People taking lithium had a thicker cortex than people with bipolar disorder who were not taking it. Other studies have found that lithium users have a larger hippocampus too.

This is a link, not proof. People who take lithium may differ in other ways as well. But it matches lab work showing that lithium helps brain cells survive and grow.

These are average differences measured across thousands of people. One scan cannot diagnose bipolar disorder in one person.

Rust on a roof

Years of rain wear metal roofing sheets thin. Sheets that were painted show far less rust. You cannot tell anything from one sheet. You can see it clearly across a whole street.

Why this matters for treatment

It is one of several reasons lithium is still the first choice for long-term treatment, more than seventy years after it was first used.

An accelerator that sticks

Evidence: proposed

This is a leading idea about what sets off the highs, and the crash that follows them.

Your reward system is what makes a goal feel worth chasing. The idea here is that in bipolar disorder this system is extra sensitive.

A goal comes into reach. It might be a new job, a deadline, or falling in love. Drive goes up. That drive brings more plans and less sleep. Less sleep raises the drive again. The loop feeds itself until it becomes hypomania or mania.

Studies support the first step. In people with bipolar disorder, events about reaching or chasing a goal predict a rise in manic symptoms. Other good news does not. In one study, young people with bipolar disorder sat end-of-year exams. They were far more likely to become hypomanic than those who did not sit them.

The same idea explains the fall. When a goal is lost or blocked, the system shuts down just as hard. That can tip a person into depression.

A bike on a steep hill

One small push and it starts rolling. Rolling makes it faster. Faster makes it harder to brake. It either runs out at the bottom, or it stops badly.

A body clock that slips

Evidence: established

The most practical finding on this page. Sleep is both an early warning and a trigger.

Every cell in your body runs on a clock of roughly 24 hours. In bipolar disorder that clock is less steady. Even between episodes, sleep and daily routine tend to be more irregular than in other people.

Losing sleep is one of the best-known triggers of mania. A few short nights can tip someone into a high. The cause might be travel, a new baby, night shifts or a deadline. Sleep also tends to shorten in the days just before mania starts.

It works in both directions. Mania cuts sleep, and lost sleep feeds mania. So a small change can grow quickly.

This is why regular sleep, wake and meal times are part of treatment. It is also why shorter sleep is often the first warning sign people learn to watch for.

A clock that runs on routine

Wind it at the same time every day and it keeps good time. Knock it around for a few nights and it starts running fast.

Why this is the useful one

Nobody can change their genes. Anybody can learn to protect their sleep and spot when it starts to shrink.

The same thing, without the vocabulary

Bipolar disorder is not simply one chemical swinging up and down. Dopamine is part of it. But the scans do not show a clean rise and fall.

It runs strongly in families, through hundreds of small genetic differences. Some of those differences are shared with depression and schizophrenia. The brain differences are small, and lithium may protect against them.

Two things often set off episodes: chasing a goal, and losing sleep. Each one can feed on itself. That is also the hopeful part. Sleep and routine are things a person can protect.