What Happens When We Sleep?
What Happens When We Sleep?
Items in this lesson
What Happens When We Sleep?
Open with the lesson's big question: sleep seems inactive from the outside, but the brain and body move through repeating stages with different levels of activity.
Learning goal
I can explain how sleep cycles move through NREM and REM stages, and what happens in each stage.
This item has no instructions
What comes to mind when you hear ‘sleep stages’ ?
Use this to surface prior ideas and misconceptions, such as ‘dreaming’, ‘deep sleep’, ‘tired’, or ‘brain resting’. Briefly connect responses to the lesson goal: sleep happens in repeating stages, not as one single state.
Sleep happens in cycles
Sleep does not happen as one unchanging state
We move through repeating sleep cycles across the night
Each cycle lasts about 90 to 120 minutes
The same broad pattern repeats several times while we sleep
Anchor the idea that the lesson is about a pattern, not a single state. Students often assume sleep simply gets deeper and stays there until morning.
How many cycles in one night?
5 to 6 hours of sleep: about 3 to 4 cycles
8 to 9 hours of sleep: about 4 to 5 cycles
The exact number varies because cycle length can range from 90 to 120 minutes
A full night's sleep means moving through these stages more than once
You can quickly connect this to students' own sleep habits, but keep the focus on the repeated structure of the night.
The four stages in each cycle
Each sleep cycle includes four stages:
NREM 1
NREM 2
NREM 3
REM
Each stage is linked to different brain activity, body changes, and psychological experiences.
Clarify that NREM means non-rapid eye movement and REM means rapid eye movement. The key idea is that stages are distinct, not interchangeable.
About how long does one sleep cycle last?
20 to 40 minutes
90 to 120 minutes
3 to 4 hours
8 to 9 hours
Distractors come from numbers students have just seen elsewhere: 3 to 4 cycles, 8 to 9 hours of sleep, and a shorter invented duration that may appeal to guessers.
Complete the summary
Sleep happens in repeating cycles across the night. Each cycle lasts about 90 to 120 minutes and includes NREM 1, NREM 2, NREM 3, and REM.
This anchors the first subtopic: sleep repeats in cycles, each with four stages. The distractors are lesson terms but only one fits each blank by meaning.
From wakefulness into sleep
When awake, the brain shows high activity
This waking activity is linked to beta waves
As sleep begins, brain activity changes rather than simply switching off
The first step into sleep is NREM 1
Keep this as a transition slide: being asleep is not the same as the brain shutting down. It shifts into a different pattern of activity.
NREM 1: very light sleep
In NREM 1, the brain produces alpha waves
This is a light stage of sleep
If someone wakes you, you might not even realise you were asleep
The body and brain are only beginning to become less responsive
Students often recognise this as the drifting-off stage. Stress that light sleep still counts as real sleep.
Progressing into deeper sleep
As a cycle continues, sleep usually moves deeper:
Awake → NREM 1 → NREM 2 → NREM 3
With each step, the brain and body become less responsive to outside stimuli, and internal activity slows further.
This slide gives the overall direction before naming the later stages in more detail.
NREM 2 and NREM 3
In NREM 2, brain activity slows further
In NREM 3, sleep is at its deepest
NREM 3 is the heaviest stage of sleep in the cycle
Someone in NREM 3 is usually much harder to wake
The source material emphasises NREM 3 as the deepest, thickest, heaviest stage. Preserve that contrast with NREM 1 and REM.
What slows down in deep sleep?
Heart rate lowers and slows down
Blood pressure lowers and slows down
Body temperature drops
Overall, the body shows reduced internal activity in deep NREM sleep
Make clear that 'deep sleep' here refers especially to NREM stages as the body becomes less active internally.
Which stage is the deepest and hardest to wake from?
NREM 1
NREM 2
NREM 3
REM
Checks the progression from light to deep sleep. A common misconception is choosing REM because it sounds active or important.
Fill in the sleep progression
When awake, the brain shows beta waves. As sleep begins, a person enters NREM 1, a very light stage linked to alpha waves. Later, sleep can deepen into NREM 3, where the body is much less responsive.
All blanks are load-bearing terms from the block on wakefulness, NREM 1, and deeper NREM sleep.
REM: dream sleep
The fourth stage is REM sleep
REM is the stage most closely linked to dreaming
Unlike deep NREM sleep, REM involves high internal activity
Sleep is not simply getting deeper all night; the pattern changes
This is the key turn in the lesson: after slowing into deep sleep, the cycle reaches a stage with active brain processes again.
Why REM is called paradoxical sleep
REM sleep is often called paradoxical sleep because two opposite things happen at once:
the brain and body show high internal activity
the large muscles are paralysed
This muscle paralysis helps prevent us from acting out dreams.
Define paradox simply as an opposition or seeming contradiction. Students should leave with the contrast: active inside, immobilised outside.
Sleepwalking is not acting out a dream
A person who is sleepwalking is not acting out a dream from REM sleep.
If they were in REM sleep, their large muscles would be paralysed. Sleepwalking points instead to a different stage of sleep.
This directly addresses the misconception from the prompt and sets up the correct stage on the next slide.
Where sleepwalking and sleep talking occur
Sleepwalking occurs in NREM 3
Sleep talking also occurs in NREM 3
This is the deep stage where the brain and body are heavily asleep internally
Because of that depth, a sleepwalker may be difficult to awaken
Reinforce the contrast: dreaming is linked to REM, but sleepwalking and sleep talking are linked to deep NREM 3.
How the night changes over time
NREM 3 gets shorter as the night goes on
Sleep cycles continue, but the balance of stages shifts
So the same four stages repeat, yet not in exactly the same proportions each time
Do not add details beyond the prompt. The essential idea is that deep NREM 3 decreases in length as the night progresses.
What sleep is really like
Sleep comes in 90 to 120 minute cycles
Each cycle includes NREM 1, NREM 2, NREM 3, and REM
NREM 3 is deepest sleep and is linked to sleepwalking and sleep talking
REM is dream sleep with muscle paralysis
Use this as the content summary before the glossary. It resolves the central question: sleep is a patterned sequence of changing states.
Why is REM called paradoxical sleep?
Dreams stop completely
Muscles grow stronger
The body cools rapidly
Active inside, still outside
Tests the core contrast: high internal activity with large-muscle paralysis. The correct option is phrased briefly but conceptually, not just as copied wording.
Why is sleepwalking not the same as acting out a dream?
Expected answer elements: sleepwalking happens in NREM 3, not REM; REM includes large-muscle paralysis, so a person would not normally act out dreams by walking around.
Complete the final recap
In REM sleep, dreaming is common and the large muscles are paralysed. NREM 3 is the deep stage linked to sleepwalking and sleep talking, and it gets shorter as the night goes on.
This recap ties together REM, NREM 3, sleepwalking, and the change across the night.
What is one thing about sleep you will remember?
Reflection prompt with no right answer. Invite students to name one clear takeaway, not to retell the whole lesson.
What helped you understand sleep cycles best today?
Reflection prompt with no right answer. Useful for identifying which representation or explanation supported understanding most.
Glossary
sleep cycle - a repeating pattern of sleep stages lasting about 90 to 120 minutes
NREM - non-rapid eye movement sleep
REM - rapid eye movement sleep, strongly linked to dreaming
NREM 1 - the first, very light stage of sleep
NREM 2 - a deeper stage after NREM 1
NREM 3 - the deepest stage of sleep
paradoxical sleep - another name for REM sleep because the brain is active while large muscles are paralysed
This item has no instructions
Glossary
beta waves - brain waves linked here to wakeful, high brain activity
alpha waves - brain waves linked here to NREM 1 sleep
muscle paralysis - the temporary inability of large muscles to move during REM sleep
sleepwalking - walking or moving during deep NREM 3 sleep
sleep talking - talking during sleep, linked here to NREM 3
This item has no instructions
