Awake
REM
Light
Deep
11 pm
12
1 am
2
3
4
5
6
7 am
one cycle ≈ 90 minutes
deep sleep comes mostly early in the night
REM grows longer towards morning
ILLUSTRATIVE NIGHT · SHAPE BASED ON TYPICAL ADULT SLEEP RECORDINGS
A
night
is
a
series
of
cycles
BRAIN WAVES (EEG)
awake: fast, small waves
deep sleep: slow, large waves
cleaning: fluid flushes out waste
measured mostly in mice: waste clears faster during sleep
deep sleep is also when most growth hormone is released, helping tissues repair
Deep
sleep
cleans
and
repairs
hippocampus
today’s memories, held briefly
cortex
long-term storage
during sleep the day’s experiences are replayed
and the important ones are strengthened in long-term memory
the rest fades
DEEP SLEEP AND REM BOTH TAKE PART · LOSING SLEEP WEAKENS NEXT-DAY RECALL
Sleep
files
the
day’s
memories
asleep
asleep
7 am
3 pm
11 pm
7 am
3 pm
11 pm
7 am
sleep pressure: adenosine builds the longer you are awake, sleep clears it
body clock: a roughly 24-hour rhythm that keeps you alert by day
you fall asleep easily when pressure is high and the clock’s alerting signal dips
CAFFEINE BLOCKS ADENOSINE’S SIGNAL · MOST ADULTS NEED 7–9 HOURS
Two
forces
decide
when
you
sleep
Why
do
we
sleep?