What Causes Nightmares and How to Stop Them

Between four and five adults in ten have had at least one nightmare in the past month, and for most of them it means very little.^[1] Having the odd nightmare is part of dreaming. It's when they come back week after week that the question of what causes nightmares starts to matter. And the answer is usually something you can find: a hard stretch at work, a new prescription, a fever, or even a habit of long lie-ins. Some people are also simply built to have more of them. Here's what the research says causes nightmares, and what you can do to have fewer.

What causes nightmares?#

Nightmares come from two things working together, how much is weighing on you and how strongly you tend to react to it. Stress, worry and trauma add to the weight, and so do things that change your sleep itself, like certain medications, stopping them suddenly, a fever or sleeping much longer than usual. How strongly you react depends partly on your genes and partly on your past.

Sleep researchers describe it in much the same terms. In one review of the evidence, chronic nightmares are what happens when a nervous system that stays on alert (even at night) meets a weakened ability to let fear fade.^[2] Normal dreaming seems to help us file frightening memories away by mixing them into new, harmless settings. But in people with frequent nightmares that filing goes wrong, and the same fearful fragments get replayed and reinforced night after night.

Nearly every cause below pushes on one side of that balance or the other. They're grouped by where they come from, starting with the ones in your waking life.

Stress, worry and trauma#

Stress on its own doesn't explain why one stressed person has nightmares every week while another sleeps fine through the same kind of month. A large survey of UK adults set out to find what predicts nightmares beyond simply feeling anxious or low, and worry stood out on its own.^[3] People who tend to turn the same problem over and over were more likely to have nightmares, and to have worse ones, even after the researchers accounted for their mood.

Pushing the worry away doesn't seem to help either. When we try to suppress a thought during the day, it's more likely to come back in our dreams, especially in REM sleep, and in one experiment a week of doing it left people sleeping worse. The evidence on this is still thin (the review that gathers it calls it the most speculative idea on its list). But if it holds, it would explain why the thing you refused to think about at 11pm is so often the thing you dream about at 5am.

Of all the causes in waking life, trauma is the strongest. About three in four people with post-traumatic stress disorder have frequent nightmares, which either replay the event or carry its fear into a different story. Those nightmares wake people up more often than ordinary ones do, with a stronger jolt of fear and helplessness. A hard childhood can leave a similar mark, and people who went through severe mistreatment as children have more frequent bad dreams and find them more distressing. If your nightmares started after something frightening happened to you, that's worth telling a doctor, because nightmares after trauma respond well to the right help.

Medications, alcohol and fever#

Some of the most common causes have nothing to do with your mood and everything to do with what's going into your body, or what has just stopped going in.

Medications, and stopping them#

If your nightmares started within days or weeks of a new prescription, the pill may well be the reason. A review of the published cases up to 1998 found that sleeping pills and sedatives, beta-blockers (the heart and blood pressure drugs) and amphetamines were the drugs most often linked to nightmares. The dopamine drugs used for Parkinson's disease also showed up.^[4] Antidepressants are more mixed. Most of them make you remember fewer dreams, but the newer ones (SSRIs and SNRIs) can make dreaming more intense, and stopping several kinds of antidepressant can bring on nightmares.^[5]

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That last point matters, because stopping a drug that holds REM sleep back has been reported to cause nightmares on its own. So don't quit a prescription to test the theory. Tell the doctor who prescribed it about the dreams and ask whether there's an alternative. Melatonin and other sleep aids, which many people blame for vivid dreams, are covered in how medication affects your dreams.

Alcohol#

Alcohol is on every list of nightmare causes, and the explanation sounds convincing. A drink before bed holds REM sleep back early in the night, and the theory goes that REM then comes back harder in the second half, with longer and more vivid dreams. But in the same UK survey, how much people drank in a week made no difference to whether they had nightmares or to how bad they were. That doesn't make a nightcap good for your sleep, but if you have a glass of wine a few evenings a week, it's probably not what's behind your nightmares. Food gets blamed too, cheese most of all, and the evidence there is just as thin (the article on diet and dreams goes through it).

Fever#

Fever is a more reliable trigger, and most of us have lived through it. In an online study, about four in five people said they had fever dreams at least some of the time when they were ill. Those dreams were stranger and more negative than their usual ones, with fewer people in them and more heat and illness.^[6] One participant dreamed of walking through a city at dusk as the air grew hotter and a ball of lava came down the mountain toward them.

The heat of the fever seems to turn into heat in the dream, and the weakness of being ill into a chase you can't win. These nightmares usually go when the fever does, so they're the one kind on this list that needs nothing from you except rest.

When your sleep is the cause#

Most of our nightmares happen in REM sleep, the stage when we have our most vivid dreams, and REM periods get longer as the night goes on (the article on what REM sleep is explains the cycle). So anything that gives you more REM in the early morning gives nightmares more room. In the UK survey, people who usually slept longer were more likely to have nightmares, even after accounting for worry, trauma and stressful events, while sleeping less made no clear difference. The authors' explanation is the extra REM that comes at the end of a long night.

Broken sleep gives nightmares another way in. Sleep apnea, where breathing stops and starts through the night, wakes you for a few seconds again and again, usually without you noticing, and it's unusually common in people with PTSD. In some studies, treating the apnea with a breathing machine reduced the nightmares as well. If you snore loudly and wake up tired however long you sleep, mention it to a doctor along with the nightmares.

Your sleeping position might matter too, though the evidence is one small study. Among 63 healthy adults, 41% of left-side sleepers said they had nightmares, against 15% of right-side sleepers, which makes it a curiosity more than a reason to change how you sleep.^[7]

Why do I have nightmares more than other people?#

Everything above can happen to anyone, which leaves the question of why some people seem to get nightmares from very little. Why does a friend sleep through a hard month while you spend it running from something? Is it just bad luck? Part of the answer is in the genes. In a study of more than 3,700 pairs of Finnish twins, identical twins were much more alike in how often they had nightmares than non-identical ones. Genes accounted for over a third of the differences between adults.^[8] The same study found that nightmares are a stable trait, so people who had a lot of them as children tended to still have them in middle age.

Researchers also point to a kind of sensitivity, a tendency to react to stress with strong negative feelings. People with frequent nightmares turn out to be more sensitive to emotional things in general, the good as well as the bad (which is a less gloomy finding than it sounds). And women report more nightmares than men, both the occasional kind and the frequent kind.

Part of that gap may be that women remember more of their dreams in general. Another part may be that depression, insomnia and PTSD, which all come with nightmares, are more common in women. In the twin study, adults with the most frequent nightmares were almost six times as likely to have had a serious psychiatric illness as those who never had any.

How to stop nightmares#

Since nightmares come from a load meeting a sensitivity, the practical steps work on the load, because that's the part you can change. Start with what changed. What was different in the weeks before they began, and did anything new go into your body? A sudden run of nightmares usually lines up with something, like a new medication or one you stopped, a hard month, an illness or a stretch of very long nights. And dealing with that one thing can be enough.

The second step is to keep your sleep regular. Waking at the same time every day, including weekends, keeps your nights from stretching into long mornings full of REM sleep. The rest of the habits that help are in the guide to better sleep. They never hurt and can only do good. If worry is what follows you to bed, try writing it down an hour before you turn the light off, along with the next thing you'll do about it. Pushing a worry away at bedtime seems to send it straight into your dreams, and a written note is a way of putting it down without pretending it isn't there.

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After that, start writing the nightmares down. It sounds like it would make them worse, but in trials, simply keeping a diary of nightmares made them come less often and feel less upsetting. A dream journal app like Oniri lets you record one by voice before you're fully awake. Over a few weeks, it shows you which nightmares keep coming back.

For a nightmare that keeps repeating, the best-studied method is imagery rehearsal. You write the nightmare down, change the story however you like (the ending in particular), then picture the new version in your head every day while you're awake. In one comparison, a detailed diary cut the number of nightmares about as much, but rewriting them did more for how frightening they felt. The steps are laid out in using dreams for healing. Some people go one step further and learn to recognize the nightmare while it's happening, which is what lucid dreaming trains.

And if the nightmares come every week, leave you dreading sleep, or started after something frightening happened, talk to a doctor. Most people never do. In surveys, fewer than a third of people with frequent nightmares believed they could be treated, and only 38% had ever told a healthcare provider about them. Frequent nightmares also go with a higher risk of suicide attempts, so if you're having thoughts of harming yourself, contact a crisis line or a doctor today.

Four or five adults in ten will have a nightmare this month and have forgotten it by lunch. If you're one of the few who has them every week, the cause is rarely hidden. It's usually something from the list above, often two or three of them at once. And most of them you can change, starting with a note of tonight's nightmare and of what changed in the weeks before they began.