start here

You go somewhere every night

Tonight your brain will run about two hours of vivid, fully furnished nonsense, with you in the lead role. By breakfast almost all of it is gone.
Over a lifetime that is roughly six years of experiences you were present for and cannot recall. Here is what is actually happening while it runs.
first, the boring truth

Sleep is not the lights going off

Your brain does not power down at night. It changes shift. Different crews clock on, doing jobs that cannot be done while you are busy walking around and having opinions.
And it does not do one long stretch of it. It works in rounds.
ONE NIGHT, FIVE CYCLES OF ABOUT 90 MINUTES
1
2
3
4
5
deep, dreamless filing dreaming
11pm. lights out. barely any dreaming yet.
THE BEST BIT IS SAVED FOR LAST
the shift pattern

Five rounds of ninety minutes

Each round has deep dreamless sleep and a dreaming slice. Early on the slice is thin. As the night goes on it gets fatter, until the last one is mostly dream.
0dreaming tonight
›››
drag through the night
👉
1953, a lab in chicago

They found it by watching eyelids

Researchers noticed sleepers' eyes flicking about under closed lids. Wake someone during the flicking and they were mid dream, every time. Wake them outside it and there was mostly nothing to report.
That is why it is called REM: rapid eye movement. It is the only outward sign of the whole performance.
what the scan shows

The picture bit is roaring

In REM the visual machinery and the emotional centres are running as hard as they do when you are awake. That is why a dream is not a vague idea. It is a place, with weather, and you are in it.
Which raises an awkward practical problem. If your brain is running a full experience, why are you not up and about performing it?
THE BODY, WHILE YOU DREAM
🔒left arm
🔒right arm
🔒left leg
🔒right leg
👀eyes
💨breathing
awake: everything you think, your body does
TWO THINGS STAY SWITCHED ON
the one rule everything hangs on

Your body gets bolted shut

As REM starts, the brainstem cuts the signal to your muscles. Arms, legs, everything, switched off at the mains. Your eyes and your breathing keep working. Nothing else does.
0muscles locked
›››
drag to bolt the doors
👉
when the bolts fail

Some people act it out

In REM sleep behaviour disorder the muscle lock does not hold, and people punch, kick, shout and leap out of bed at whatever their dream is doing. Partners get hurt. So do they.
It is not a quirk. In many people it turns up years before Parkinson's or a related condition, which is why doctors take it seriously rather than laughing at it.
the second thing that switches off

The critic is sent home

The front of your brain, the part that plans, checks and says hang on, that makes no sense, goes quiet in REM. Nobody is left in the room to object.
So the plot is accepted exactly as delivered. Your dead grandmother is driving the bus. Fine. You have an exam you never enrolled for. Fine.
TWO DIALS, MOVING OPPOSITE WAYS
the part that says
"hang on, that is nonsense"
100%
pictures and feelings
running the show
20%
"I am late for a train"
AND IT ALL SEEMED FINE AT THE TIME
watch the two dials

Logic down, pictures up

Turn the checker down and turn the imagery and feeling up, and you get the exact flavour of a dream: completely convincing while it runs, obviously mad the second you describe it out loud.
0sense checking
›››
drag the critic out
👉
so what is it for

Yesterday gets filed overnight

While you sleep your brain replays the day at high speed and moves what matters into long term storage. Learn something, then sleep on it, and you genuinely do it better tomorrow. Skip the sleep and you do not.
Dreaming looks like part of that sorting: loose ends, worries and half solved problems being run through the machine, which is why the cast is so often people and places from your actual week.
the missing tape

You forget because the recorder is off

One of the chemicals your brain needs to lay down new memories is almost entirely absent during REM. The dream happens in full. Nothing is writing it down.
That is why a dream survives only if you wake up inside it, and why it drains away in seconds if you get up and start your day instead of lying still.
the unpleasant ones

Bad dreams are rehearsals

A nightmare is your brain running a threat with the volume up and no real risk attached. One decent theory is that it is practice, another is that it takes the sting out of memories by replaying them without the panic chemicals.
When that process jams, as it does in trauma, the same nightmare loops instead of settling. There is a treatment for it: you rehearse a new ending while awake, night after night, until the dream takes the note.
the strange bit

You can hold a conversation from inside one

In 2021 labs in four countries asked sleeping people simple sums and got correct answers back, signalled with eye movements and facial twitches, while they were still dreaming.
It is a real two way channel into a running dream. Not folklore, replicated across four independent teams.
the other failure mode

Sometimes you wake up before the bolts do

Sleep paralysis is the muscle lock still on after your mind has come back. You are awake, aware, and cannot move a finger, often with a dream image still hanging in the room.
Terrifying, harmless, and over in a minute or two. It is the same safety mechanism, just clocking off late.
what to do with all this

Your alarm eats the best cycle

Dreaming is stacked at the end of the night, so an hour cut off the morning costs you far more dreaming than an hour cut off the start. A nightcap makes it worse: alcohol flattens REM early on, then it comes back rough later.
If you want to remember one, do not move when you wake. Lie still, replay it once, then write it down. Move first and it is gone.
the whole thing

Seven things you now know

  • 2 hoursOf dreaming a night, in slices, most of it near morning.
  • 90 minPer cycle, five rounds, dreaming slice fattens each time.
  • REMEyes flicking is the only sign from outside.
  • boltedMuscles cut off at the mains, so you cannot act it out.
  • critic outThe checking part goes quiet, so any plot passes.
  • no tapeThe memory chemical is absent, so it does not get saved.
  • filingBest guess at the point: sorting yesterday and defusing it.
Not your brain switched off. Your brain running the night shift, with the doors bolted and nobody taking minutes.
a prune · regent