Why we sleep & dream

Without sleep, the brain can’t form or maintain the pathways that let you learn and make new memories. More on this in the memory section.

The “synaptic homeostasis” idea suggests that while we are awake, learning strengthens connections all over the brain, which uses energy and gets crowded. During sleep, the brain may scale these connections back down, keeping the important ones.

Researchers propose that sleep, and especially REM sleep, helps keep the emotional brain in balance, getting us ready for the social and emotional demands of the next day.

NINDS notes that recent findings suggest sleep plays a housekeeping role, removing toxins that build up while we are awake. Scientists are still debating how this works.

The hypothalamus has groups of nerve cells that act as control centers for sleep and waking. Inside it is the body’s master clock, which gets information about light straight from the eyes.

The brainstem controls the switch between wake and sleep. During REM sleep it also sends signals that relax the muscles, so we don’t act out our dreams.

The thalamus passes information from the senses to the outer layer of the brain. In most sleep it goes quiet, letting you tune out the world. In REM sleep it becomes active, sending images, sounds and other sensations that fill our dreams.

The pineal gland takes signals from the master clock and releases a hormone that helps you fall asleep once it gets dark. Its daily rise and fall helps keep your body in step with day and night.

As you stay awake, cells release adenosine, which helps make you feel sleepy. Caffeine works by blocking adenosine, which is why it can hide tiredness for a while.

The amygdala, an almond-shaped area involved in emotions, becomes more and more active during REM sleep. This may be one reason dreams can feel so emotional.

In a brain-imaging study, one night without sleep weakened activity in the hippocampus, a key memory area, while people tried to learn. They remembered less afterward.

Newer research describes the sleeping brain as tuned for strengthening memories, while the waking brain is tuned for taking them in. Memory consolidation most likely needs both non-REM and REM sleep.

In one study, people learned a virtual maze, then napped. Those who dreamed about the maze did much better when tested again. Simply thinking about it while awake didn’t give the same boost.

Ideas

• Keep a regular sleep schedule during exams or busy work weeks
• Practice a new skill, then get a full night’s sleep
• Review important things in the evening, not in the middle of the night
• Spread learning over several days so sleep can help each time

After one night without sleep, the amygdala was about 60% more reactive to upsetting pictures than after a normal night. Its links with the front of the brain, which helps calm it, were weaker.

In a lab study, young adults kept awake all night were less accurate at judging angry and happy faces. One night of recovery sleep fixed it.

During REM sleep, levels of noradrenaline, a stress-related brain chemical, fall lower than at any other time of day. Researchers propose this calm chemical state lets the brain revisit emotional memories while easing their emotional charge.

They describe it as sleeping to remember the experience, but to forget its emotional tone. This idea is still being tested.

The Sleep, mood & depression guide goes deeper on sleep and low mood. The How emotions work guide explains what feelings are and how they work in the body and brain.

In a study of 20 healthy adults, one night without sleep was linked with more beta-amyloid, a protein tied to Alzheimer’s disease, in parts of the brain. The increase went along with worse mood.

In 2024, a team in the UK measured clearance in mouse brains and found it was lower, not higher, during sleep. Scientists are now working out why studies disagree.

This is how science works: big ideas get tested again and again. What isn’t in doubt is that sleep matters a great deal for the brain.

Dreaming was long linked with REM sleep, but it happens in non-REM sleep too. In one study, scientists could predict whether someone was dreaming, in either kind of sleep, by watching a “hot zone” toward the back of the brain.

In REM sleep, your arm and leg muscles become temporarily paralyzed. This stops you from acting out your dreams.

Events from the day often find their way into dreams, and people under stress or with anxiety are more likely to have frightening dreams.

Researchers call this the “continuity” idea: our dreams tend to continue the worries, interests and feelings of waking life.

Some people dream in colour, while others recall dreams only in black and white.

People who are blind dream too. People born blind report more sounds, touch, taste and smell in their dreams than sighted people do.

Dreams are strongly linked to emotion, and the brain areas active in emotional dreams overlap with those that handle feelings when awake. Some researchers think dreaming helps take the edge off painful memories.

Dreaming about something you just learned has been linked with remembering it better, which suggests dreams may reflect the brain replaying and storing new memories.

The “threat simulation” theory suggests dreams evolved to rehearse dangerous situations, so our ancestors could practise spotting and escaping threats.

An influential 1977 theory, called activation-synthesis, proposed that the brainstem sends bursts of activity during REM sleep, and the rest of the brain weaves them into a story using stored memories.

One leading idea is that we need to wake briefly during the night to store a dream in memory. In one study, people who often recalled dreams reacted more strongly to sounds during sleep and while awake than people who rarely did.

A review of 50 years of research found that about 55% of people have had at least one lucid dream, and about 23% have them at least monthly.

Researchers have also found that lucid dreaming is linked to fewer nightmares, and are studying it as a possible way to help.

About 7.6% of people have had sleep paralysis at least once, and it is more common in students. You can’t move your muscles while waking up or falling asleep, because your body is still in sleep mode while your brain is active. It can be scary, but it is harmless.

People with stress or anxiety are more likely to have frightening dreams. Rehearsing a new version of a frightening dream while awake is a promising way to change nightmares.

Your brain and body stay remarkably active during sleep. In REM sleep, brain waves look much closer to waking, and breathing, heart rate and blood pressure rise to near-waking levels.

Everyone dreams, but many people don’t remember most of their dreams.

Dreams can happen in all stages of sleep, though they tend to be most vivid in REM sleep.

One night without sleep weakened the brain’s memory system during learning, and people remembered less.

Depending on how short on sleep you are, sleeping longer at weekends may not be enough to replace the sleep you’ve missed.

People born blind have dreams full of sound, touch, taste and smell. Their dreams carry the same kinds of emotions and themes as sighted people’s dreams.

Telling a partner, friend or therapist about a vivid or upsetting dream can help you notice what’s on your mind. With children, asking “What was your dream about?” at breakfast can be an easy way into talking about feelings.

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