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Some mornings when you wake up, your head is fuzzy, your body is heavy, and you don’t feel rested. It felt like you were dreaming all night.

But did all that dreaming actually wear you out? Let’s look at what the science says.

We all dream, but not everyone remembers it

Most dreaming occurs during rapid eye movement (REM) sleep, which makes up 20–25% of our total sleep time.

We have four to six rounds of REM throughout the night, with each round growing longer as morning approaches. We all dream, and most of us dream multiple times a night, whether we remember it or not.

If you wake up during or just after a REM period, you are more likely to remember what you were dreaming.

Whether you remember a dream can also depend on the emotional intensity of the dream and whether you briefly wake up in the night, as well as differences in how individual brains store memories overnight.

People who regularly remember vivid, emotionally intense dreams tend to have lighter, more broken sleep.

What happens in your brain when you dream?

During REM sleep, your brain is running almost as hard as it does when you are awake, firing away, while your body lies completely still. Your muscles are essentially paralysed, which stops you acting out what’s happening in the dream.

At the same time, the parts of the brain that handle emotion – the amygdala, hippocampus and thalamus – are highly active. The prefrontal cortex, which normally keeps things rational and logical, is much less engaged.

So you get vivid, emotionally charged experiences that feel completely real but make no logical sense. That part is normal.

How long do dreams last? And are we any good at judging?

Most people assume dreams are brief, fragmented flashes.

In fact, the evidence suggests otherwise. REM sleep dreams appear to unfold roughly in real time.

When researchers have woken people from REM sleep and asked them to describe their dream, the length of their account closely matches the duration spent in the dreaming stage of sleep (REM episode) . A dream that feels like 20 minutes was probably about that long in real life.

Where people go wrong is estimating how much of the whole night they spent dreaming. A stressful or vivid dream feels longer and stays with you. A dull one vanishes before you even open your eyes.

On top of that, we mostly remember dreams we actually woke up during.

Someone who was sure they dreamed all night probably had a completely normal night of REM sleep. They just happened to wake during the emotionally charged parts, and those are the ones that stuck.

So does dreaming itself actually tire you out?

During REM sleep, your brain isn’t resting in the way deep sleep allows. Even so, brain imaging studies suggest this energy use alone doesn’t account for the fatigue people feel after a heavy night of dreaming.

Dreaming on its own does not seem to impact your sleep quality unless it tips into nightmares.

The more straightforward explanation is this: if you remember a dream, you almost certainly woke up during it. Those wake-ups, even the ones you barely register, take time away from deep sleep.

These wake-ups also give the brain less opportunity to clear a waste product called adenosine. During the day, adenosine builds up in the brain. As it accumulates, the pressure to sleep grows. One of sleep’s main jobs is to flush this out, and it does that most effectively during deep sleep. Wake up before it’s done and you might find yourself more tired the next day.

Waking from REM sleep is also harder on the body than waking from lighter stages. It can produce sleep inertia, that thick, foggy state in which your brain refuses to come online. The tiredness is not a consequence of dreaming: it’s a consequence of when you woke up and what stage you were pulled from.

Consider the quality of your sleep

When sleep is cut short or is repeatedly broken, the brain makes up for lost REM time on subsequent nights, spending a higher proportion of sleep in that stage. This is called REM rebound.

REM rebound is a compensatory response rather than a problem in itself. The actual problem is whatever is causing the sleep disruption.

If you regularly remember most of your dreams, feel like the number of dreams you have has increased, or find yourself waking up tired most mornings, your fragmented sleep may mean the brain isn’t getting the deep, restorative stages it needs.

If this describes you, and it affects how you feel and function through the day, it’s worth having a conversation with your doctor.

Yaqoot Fatima receives funding from NHMRC, MRFF and Beyond Blue. She is affiliated with Sleep Health Foundation

Danielle Wilson and Nisreen Aouira do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.