Why We Sleep

Part of the reason the heart suffers so dramatically under the weight of sleep
deprivation concerns blood pressure. Have a quick look at your right forearm and
pick out some veins. If you wrap your left hand around that forearm, just below
the elbow, and grip it, like a tourniquet, you will see those vessels start to balloon.
A little alarming, isn’t it? The ease with which just a little sleep loss can pump up
pressure  in  the  veins  of  your  entire  body,  stretching  and  distressing  the  vessel
walls, is equally alarming. High blood pressure is so common nowadays that we
forget  the  deathly  toll  it  inflicts.  This  year  alone,  hypertension  will  steal  more
than  7  million  people’s  lives  by  way  of  cardiac  failure,  ischemic  heart  disease,
stroke,  or  kidney  failure.  Deficient  sleep  is  responsible  for  many  of  these  lost
fathers, mothers, grandparents, and beloved friends.
As with other consequences of sleep loss we’ve encountered, you don’t need a
full  night  of  total  sleep  deprivation  to  inflict  a  measurable  impact  on  your
cardiovascular  system.  One  night  of  modest  sleep  reduction—even  just  one  or
two  hours—will  promptly  speed  the  contracting  rate  of  a  person’s  heart,  hour
upon  hour,  and  significantly  increase  the  systolic  blood  pressure  within  their
vasculature.
I
 You  will  find  no  solace  in  the  fact  that  these  experiments  were
conducted  in  young,  fit  individuals,  all  of  whom  started  out  with  an  otherwise
healthy cardiovascular system just hours before. Such physical fitness proves no
match for a short night of sleep; it affords no resistance.

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Beyond accelerating your heart rate and increasing your blood pressure, a lack
of sleep further erodes the fabric of those strained blood vessels, especially those
that feed the heart itself, called the coronary arteries. These corridors of life need
to be clean and open wide to supply your heart with blood at all times. Narrow or
block  those  passageways,  and  your  heart  can  suffer  a  comprehensive  and  often
fatal attack caused by blood oxygen starvation, colloquially known as a “massive
coronary.”
One cause of a coronary artery blockage is atherosclerosis, or the furring up of
those  heart  corridors  with  hardened  plaques  that  contain  calcium  deposits.
Researchers  at  the  University  of  Chicago  studied  almost  five  hundred  healthy
midlife  adults,  none  of  whom  had  any  existing  heart  disease  or  signs  of
atherosclerosis.  They  tracked  the  health  of  the  coronary  arteries  of  these
participants for a number of years, all the while assessing their sleep. If you were
one of the individuals who were obtaining just five to six hours each night or less,
you  were  200  to  300  percent  more  likely  to  suffer  calcification  of  your  coronary
arteries  over  the  next  five  years,  relative  to  those  individuals  sleeping  seven  to
eight hours. The deficient sleep of those individuals was associated with a closing
off of the critical passageways that should otherwise be wide open and feeding the
heart  with  blood,  starving  it  and  significantly  increasing  the  risk  of  a  coronary
heart attack.
Although the mechanisms by which sleep deprivation degrades cardiovascular
health are numerous, they all appear to cluster around a common culprit, called
the  sympathetic  nervous  system.  Abandon  any  thoughts  of  love  or  serene
compassion based on the misguiding name. The sympathetic nervous system is
resolutely  activating,  inciting,  even  agitating.  If  needed,  it  will  mobilize  the
evolutionarily  ancient  fight-or-flight  stress  response  within  the  body,
comprehensively  and  in  a  matter  of  seconds.  Like  an  accomplished  general  in
command of a vast military, the sympathetic nervous system can muster activity
in  a  vast  assortment  of  the  body’s  physiological  divisions—from  respiration,
immune function, and stress chemicals to blood pressure and heart rate.
An  acute  stress  response  from  the  sympathetic  nervous  system,  which  is
normally only deployed for short periods of time lasting minutes to hours, can be
highly adaptive under conditions of credible threat, such as the potential of real
physical  attack.  Survival  is  the  goal,  and  these  responses  promote  immediate
action to accomplish just that. But leave that system stuck in the “on” position for
long durations of time, and sympathetic activation becomes deeply maladaptive.
In fact, it is a killer.

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With  few  exceptions  over  the  past  half  century,  every  experiment  that  has
investigated  the  impact  of  deficient  sleep  on  the  human  body  has  observed  an
overactive  sympathetic  nervous  system.  For  as  long  as  the  state  of  insufficient
sleep lasts, and for some time thereafter, the body remains stuck in some degree
of  a  fight-or-flight  state.  It  can  last  for  years  in  those  with  an  untreated  sleep
disorder, excessive work hours that limit sleep or its quality, or the simple neglect
of sleep by an individual. Like a car engine that is revved to a shrieking extreme
for  sustained  periods  of  time,  your  sympathetic  nervous  system  is  floored  into
perpetual overdrive by a lack of sleep. The consequential strain that is placed on
your  body  by  the  persistent  force  of  sympathetic  activation  will  leak  out  in  all
manner of health issues, just like the failed pistons, gaskets, seals, and gnashing
gears of an abused car engine.
Through  this  central  pathway  of  an  overactive  sympathetic  nervous  system,
sleep  deprivation  triggers  a  domino  effect  that  will  spread  like  a  wave  of  health
damage throughout your body. It starts with removing a default resting brake that
normally  prevents  your  heart  from  accelerating  in  its  rate  of  contraction.  Once
this brake is released, you will experience sustained speeds of cardiac beating.
As your sleep-deprived heart beats faster, the volumetric rate of blood pumped
through your vasculature increases, and with that comes the hypertensive state of
your blood pressure. Occurring at the same time is a chronic increase in a stress
hormone called cortisol, which is triggered by the overactive sympathetic nervous
system.  One  undesirable  consequence  of  the  sustained  deluge  of  cortisol  is  the
constriction of those blood vessels, triggering an even greater increase in blood
pressure.
Making  matters  worse,  growth  hormone—a  great  healer  of  the  body—which
normally  surges  at  night,  is  shut  off  by  the  state  of  sleep  deprivation.  Without
growth  hormone  to  replenish  the  lining  of  your  blood  vessels,  called  the
endothelium,  they  will  be  slowly  shorn  and  stripped  of  their  integrity.  Adding
insult to real injury, the hypertensive strain that sleep deprivation places on your
vasculature  means  that  you  can  no  longer  repair  those  fracturing  vessels
effectively.  The  damaged  and  weakened  state  of  vascular  plumbing  throughout
your  body  now  becomes  systemically  more  prone  to  atherosclerosis  (arteries
furring  up).  Vessels  will  rupture.  It  is  a  powder  keg  of  factors,  with  heart  attack
and stroke being the most common casualties in the explosive aftermath.
Compare this cascade of harm to the healing benefits that a full night of sleep
normally  lavishes  on  the  cardiovascular  system.  During  deep  NREM  sleep
specifically,  the  brain  communicates  a  calming  signal  to  the  fight-or-flight

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sympathetic branch of the body’s nervous system, and does so for long durations
of  the  night.  As  a  result,  deep  sleep  prevents  an  escalation  of  this  physiological
stress  that  is  synonymous  with  increased  blood  pressure,  heart  attack,  heart
failure, and stroke. This includes a calming effect on the contracting speed of your
heart.  Think  of  your  deep  NREM  sleep  as  a  natural  form  of  nighttime  blood-
pressure management—one that averts hypertension and stroke.
When communicating science to the general public in lectures or writing, I’m
always  wary  of  bombarding  an  audience  with  never-ending  mortality  and
morbidity statistics, lest they themselves lose the will to live in front of me. It is
hard  not  to  do  so  with  such  compelling  masses  of  studies  in  the  field  of  sleep
deprivation. Often, however, a single astonishing result is all that people need to
apprehend the point. For cardiovascular health, I believe that finding comes from
a “global experiment” in which 1.5 billion people are forced to reduce their sleep
by one hour or less for a single night each year. It is very likely that you have been