Cognitive Science 101Lecture 7

How sleep shapes memory

What the sleeping brain does with everything you learned today.

Department of PsychologyAutumn term

Today

Four questions, twenty minutes

  1. Why do we forget? 4 min
  2. What happens while we sleep? 6 min
  3. What is the evidence? 6 min
  4. What should students do? 4 min

01

Why do we forget?

Memories start fragile. Most of what we learn fades unless something stabilises it.

Consolidation

/kənˌsɒlɪˈdeɪʃən/ · noun

The process that turns a fragile new memory into a stable, long-term one — largely while we sleep.

“Without consolidation, a lecture is a story you heard once.”

Sleep after learning protects memories from being overwritten by the rest of the day.

Jenkins & Dallenbach, summarising their 1924 finding

02

What happens while we sleep?

One sleep cycle · about 90 minutes

The night runs in repeating cycles

  1. N1 · Drifting

    Light sleep, a few minutes long. Easy to wake from.

  2. N2 · Spindles

    Bursts of brain activity called sleep spindles help store new skills and facts.

  3. N3 · Deep sleep

    Slow waves dominate. The hippocampus replays the day’s experiences.

  4. REM · Dreaming

    Vivid dreams. Linked to emotional memory and connecting ideas.

A typical adult night

Half the night is stage N2

Deep sleep comes early; REM grows longer toward morning.

Cutting the last two hours costs you mostly REM.

Approximate textbook proportions for healthy adults.

Reading a hypnogram

One night, about five cycles

Hypnogram of one night: sleep stages from 11 pm to 7 am, with deep sleep early and longer REM toward morning
  1. Deep sleep. Front-loaded into the first cycles.
  2. First REM. Arrives after about 90 minutes.
  3. Brief waking. Normal, and rarely remembered.
  4. Longest REM. Comes just before waking.

REM across the night

The last cycle holds the most REM

  1. REM gets longer with every cycle.
  2. Cycle 5 has four times the REM of cycle 1.
  3. Almost a third of the night's REM.

The two-stage model

From fast learner to long-term store

ExperienceLecture, practice, reading
HippocampusFast, temporary encoding
Sleep replayPatterns reactivated in N3
NeocortexSlow, durable storage
Abstract network of connected neurons

Replay

The brain rehearses the day at high speed

In 1994, Wilson and McNaughton recorded rats’ place cells. The cells that fired while the rats ran a maze fired again, in the same order, during their sleep afterwards.

03

What is the evidence?

A century of findings

How the idea took shape

  1. 1885

    Forgetting curve

    Ebbinghaus measures how fast memories fade.

  2. 1924

    Sleep protects

    Jenkins & Dallenbach: less forgetting after sleep.

  3. 1953

    REM discovered

    Aserinsky & Kleitman identify REM sleep.

  4. 1994

    Replay

    Place cells re-fire during sleep.

  5. 2002

    Skills improve

    Motor skills sharpen after a night’s sleep.

Same material, different night

All-nighter vs. a full night

Pulling an all-nighter

  • More hours with the material
  • Little deep sleep or REM to consolidate it
  • Tired brain at exam time
vs

Studying, then sleeping

  • Fewer hours of study
  • Full cycles of replay overnight
  • Rested and able to recall

Short on sleep

1 in 3

US adults regularly sleep less than the recommended seven hours.

Source: US Centers for Disease Control and Prevention.

The numbers to remember

Sleep by the numbers

7+

hours recommended for adults each night

~90

minutes in one full sleep cycle

4–6

cycles in a healthy night

1 in 3

adults who fall short

Stage by stage

What each stage contributes

StageBrain activityLinked toShare of night
N1Slowing alpha to theta wavesFalling asleep~5%
N2Sleep spindles, K-complexesSkills and facts~50%
N3Slow, large delta wavesReplay, fact memory~20%
REMFast, wake-like activityEmotion, linking ideas~25%

04

What should students do?

The usual plan

Late review fades before the exam

The problem

Review at midnight, test at nine

The material never gets a full night of slow-wave and REM sleep to settle in.

The fix

Review, sleep, then recall

Study in the evening, sleep a full night, and test yourself over breakfast.

Effort vs. payoff

Sleep is the cheapest study tool

Where common study habits land for memory.

Sleep after study

Free, and it doubles as consolidation.

Spaced practice

Strong gains, but needs a plan.

Rereading notes

Feels familiar, fades fast.

All-nighters

Hours spent, recall lost.

Effort

Payoff

The night before an exam

Pick a sleep plan, not a cram plan

Minimum

6hours

Four cycles. Some REM lost.

  • Lights out by 1 am
  • Skip the last review
Recommended

Full night

8hours

Five cycles, full morning REM.

  • Lights out by 11 pm
  • Review hard topics first
  • Recall test at breakfast

Recovery

9hours

After a short week of sleep.

  • Lights out by 10 pm
  • No screens in bed

Exam fortnight

A two-week plan with sleep built in

  1. Week 1
  2. Week 2
  3. Exam eve
  4. Exam day

Learn

New topics, one a day

Fill the gaps

Review

Spaced recall, every 2 days

Light review only

Sleep

7–9 hours, fixed bedtime

Wake rested

Practical advice

Study with sleep, not against it

  1. Study, then sleep. Review hard material in the evening, then let the night consolidate it.
  2. Spread it out. Several sessions with sleep in between beat one long cram.
  3. Protect the morning. The last hours of sleep carry most of your REM.

Try this week

Three small experiments

01

Evening review

Spend 15 minutes re-reading today’s notes before bed.

02

Fixed wake time

Get up at the same time every day, weekends included.

03

Test yourself

Next morning, write down what you remember without notes.

For the programmers

Spacing your reviews

A tiny scheduler: each review doubles the gap.

def review_days(first=1, reviews=5):
    # sleep happens between every review
    day, gap = 0, first
    for _ in range(reviews):
        day += gap
        yield day
        gap *= 2

print(list(review_days()))  # [1, 3, 7, 15, 31]

The best study session ends with going to bed.

Further reading

Three places to go deeper

Abstract warm wave pattern
Matthew Walker, Why We Sleep (2017)
Abstract green wave pattern
Rasch & Born, “About Sleep’s Role in Memory” (2013)
Abstract rose wave pattern
Stickgold, “Sleep-dependent memory consolidation” (2005)

Teaching team

Questions? Find us here

A
Course leadOffice hours Tue 2–4 pm
T
Teaching assistantLab sessions Thu
S
Study supportDrop-in, library level 2

Tonight’s homework

Go to sleep. Your memory is doing the work.

Recap

The night at a glance

Sleep turns today’s study into memory

Deep sleep replays the day. REM helps connect it to what you already know.

Abstract rose wave pattern

7+h

A night, for adults

4–6

Cycles in a healthy night

Sleep

Cognitive Science 101Lecture 7 · Take-home issue

What your brain does after lights out

  • 90 minOne full cycle of sleep stages
  • Deep sleepWhere the day gets replayed
  • 3 habitsTo protect tonight’s learning

Thank you.

  • SlidesCourse page → Week 7
  • ReadingChapter 9, pp. 212–240
  • Next weekAttention and distraction