Why We Sleep

Participants in this experiment again underwent two different conditions, acting
as their own baseline control: four nights of eight and a half hours’ time in bed,
and four nights of four and a half hours’ time in bed. Each day, participants were
limited  to  the  same  level  of  physical  activity  under  both  conditions.  Each  day,
they were given free access to food, and the researchers meticulously counted the
difference in calorie consumption between the two experimental manipulations.
When short sleeping, the very same individuals ate 300 calories more each day
—or  well  over  1,000  calories  before  the  end  of  the  experiment—compared  to
when they were routinely getting a full night of sleep. Similar changes occur if you
give  people  five  to  six  hours  of  sleep  over  a  ten-day  period.  Scale  that  up  to  a
working year, and assuming one month of vacation in which sleep miraculously
becomes  abundant,  and  you  will  still  have  consumed  more  than  70,000  extra
calories. Based on caloric estimates, that would cause 10 to 15 pounds of weight
gain a year, each and every year (which may sound painfully familiar to many of
us).
Van Cauter’s next experiment was the most surprising (and devilish) of all. Fit,
healthy individuals went through the same two different conditions as before: four
nights  of  eight  and  a  half  hours’  time  in  bed,  and  four  nights  of  four  and  a  half
hours’  time  in  bed.  However,  on  the  last  day  if  each  of  the  experimental
conditions, something different happened. Participants were offered an additional
food  buffet  stretched  across  a  four-hour  period.  Set  out  in  front  of  them  was  an
assortment  of  foods,  from  meats,  vegetables,  bread,  potatoes,  and  salad  to  fruit
and  ice  cream.  Set  to  one  side,  however,  was  access  to  a  bonus  snack  bar  filled
with cookies, chocolate bars, chips, and pretzels. Participants could eat as much

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as  they  wanted  in  the  four-hour  period,  with  the  buffet  even  being  replenished
halfway  through.  Importantly,  the  subjects  ate  alone,  limiting  social  or
stigmatizing influences that could alter their natural eating urges.
Following  the  buffet,  Van  Cauter  and  her  team  once  again  quantified  what
participants  ate,  and  how  much  they  ate.  Despite  eating  almost  2,000  calories
during the buffet lunch, sleep-deprived participants dove into the snack bar. They
consumed an additional 330 calories of snack foods after the full meal, compared
to when they were getting plenty of sleep each night.
Of  relevance  to  this  behavior  is  a  recent  discovery  that  sleep  loss  increases
levels of circulating endocannabinoids, which, as you may have guessed from the
name,  are  chemicals  produced  by  the  body  that  are  very  similar  to  the  drug
cannabis.  Like  marijuana  use,  these  chemicals  stimulate  appetite  and  increase
your desire to snack, otherwise known as having the munchies.
Combine  this  increase  in  endocannabinoids  with  alterations  in  leptin  and
ghrelin  caused  by  sleep  deprivation  and  you  have  a  potent  brew  of  chemical
messages all driving you in one direction: overeating.
Some  argue  that  we  eat  more  when  we  are  sleep-deprived  because  we  burn
extra calories when we stay awake. Sadly, this is not true. In the sleep-restriction
experiments  described  above,  there  are  no  differences  in  caloric  expenditure
between  the  two  conditions.  Take  it  to  the  extreme  by  sleep-depriving  an
individual  for  twenty-four  hours  straight  and  they  will  only  burn  an  extra  147
calories,  relative  to  a  twenty-four-hour  period  containing  a  full  eight  hours  of
sleep. Sleep, it turns out, is an intensely metabolically active state for brain and
body  alike.  For  this  reason,  theories  proposing  that  we  sleep  to  conserve  large
amounts  of  energy  are  no  longer  entertained.  The  paltry  caloric  savings  are
insufficient to outweigh the survival dangers and disadvantages associated with
falling asleep.
More  importantly,  the  extra  calories  that  you  eat  when  sleep-deprived  far
outweigh  any  nominal  extra  energy  you  burn  while  remaining  awake.  Making
matters worse, the less an individual sleeps, the less energy he or she feels they
have, and the more sedentary and less willing to exercise they are in real-world
settings. Inadequate sleep is the perfect recipe for obesity: greater calorie intake,
lower calorie expenditure.
Weight gain caused by short sleep is not just a matter of eating more, but also
a change in what you binge eat. Looking across the different studies, Van Cauter
noticed that cravings for sweets (e.g., cookies, chocolate, and ice cream), heavy-
hitting  carbohydrate-rich  foods  (e.g.,  bread  and  pasta),  and  salty  snacks  (e.g.,

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potato  chips  and  pretzels)  all  increased  by  30  to  40  percent  when  sleep  was
reduced  by  several  hours  each  night.  Less  affected  were  protein-rich  foods  (e.g.,
meat and fish), dairy items (such as yogurt and cheese), and fatty foods, showing a
10 to 15 percent increase in preference by the sleepy participants.
Why is it that we lust after quick-fix sugars and complex carbohydrates when
sleep-deprived? My research team and I decided to conduct a study in which we
scanned  people’s  brains  while  they  were  viewing  and  choosing  food  items,  and
then  rated  how  much  they  desired  each  one.  We  hypothesized  that  changes
within the brain may help explain this unhealthy shift in food preference caused
by  a  lack  of  sleep.  Was  there  a  breakdown  in  impulse-control  regions  that
normally  keep  our  basic  hedonic  food  desires  in  check,  making  us  reach  for
doughnuts or pizza rather than whole grains and leafy greens?
Healthy,  average-weight  participants  performed  the  experiment  twice:  once
when  they  had  had  a  full  night  of  sleep,  and  once  after  they  had  been  sleep-
deprived for a night. In each of the two conditions they viewed eighty similar food
images,  ranging  from  fruits  and  vegetables,  such  as  strawberries,  apples,  and
carrots, to high-calorie items, such as ice cream, pasta, and doughnuts. To ensure
that  participants  were  making  choices  that  reflected  their  true  cravings  rather
than  simply  choosing  items  that  they  thought  would  be  the  right  or  most
appropriate  choice,  we  forced  an  incentive:  after  they  came  out  of  the  MRI
machine, we gave them a serving of the food they told us they most craved during
the task, and politely asked them to eat it!
Comparing  the  patterns  of  brain  activity  between  the  two  conditions  within
the  same  individual,  we  discovered  that  supervisory  regions  in  the  prefrontal
cortex  required  for  thoughtful  judgments  and  controlled  decisions  had  been
silenced  in  their  activity  by  a  lack  of  sleep.  In  contrast,  the  more  primal  deep-
brain structures that drive motivations and desire were amplified in response to
the food images. This shift to a more primitive pattern of brain activity without
deliberative control came with a change in the participants’ food choices. High-
calorie foods became significantly more desirable in the eyes of the participants
when sleep-deprived.  When we  tallied  up the  extra  food items  that  participants
wanted when they were sleep-deprived, it amounted to an extra 600 calories.
The encouraging news is that getting enough sleep will help you control body
weight.  We  found  that  a  full  night  of  sleep  repairs  the  communication  pathway
between  deep-brain  areas  that  unleash  hedonic  desires  and  higher-order  brain
regions whose job it is to rein in these cravings. Ample sleep can therefore restore

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a system of impulse control within your brain, putting the appropriate brakes on
potentially excessive eating.
South of the brain, we are also discovering that plentiful sleep makes your gut
happier.  Sleep’s  role  in  redressing  the  balance  of  the  body’s  nervous  system,
especially  its  calming  of  the  fight-or-flight  sympathetic  branch,  improves  the
bacterial  community  known  as  your  microbiome,  which  is  located  in  your  gut
(also  known  as  the  enteric  nervous  system).  As  we  learned  about  earlier,  when
you do not get enough sleep, and the body’s stress-related, fight-or-flight nervous
system is revved up, this triggers an excess of circulating cortisol that cultivates
“bad bacteria” to fester throughout your microbiome. As a result, insufficient sleep
will  prevent  the  meaningful  absorption  of  all  food  nutrients  and  cause
gastrointestinal problems.
III
Of course, the obesity epidemic that has engulfed large portions of the world is
not caused by lack of sleep alone. The rise in consumption of processed foods, an
increase in serving sizes, and the more sedentary nature of human beings are all
triggers.  However,  these  changes  are  insufficient  to  explain  the  dramatic
escalation of obesity. Other factors must be at play.
Based  on  evidence  gathered  over  the  past  three  decades,  the  epidemic  of
insufficient  sleep  is  very  likely  a  key  contributor  to  the  epidemic  of  obesity.
Epidemiological studies have established that people who sleep less are the same
individuals who are more likely to be overweight or obese. Indeed, if you simply
plot the reduction in sleep time (dotted line) over the past fifty years on the same
graph as the rise in obesity rates across the same time period (solid line), shown
in
Figure 13
, the data infer this relationship clearly.