Why We Sleep

Figure 6: The Urge to Sleep
What  happens  to  all  of  the  accumulated  adenosine  once  you  do  fall  asleep?
During  sleep,  a  mass  evacuation  gets  under  way  as  the  brain  has  the  chance  to
degrade  and  remove  the  day’s  adenosine.  Across  the  night,  sleep  lifts  the  heavy

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weight of sleep pressure, lightening the adenosine load. After approximately eight
hours of healthy sleep in an adult, the adenosine purge is complete. Just as this
process  is  ending,  the  marching  band  of  your  circadian  activity  rhythm  has
fortuitously returned, and its energizing influence starts to approach. When these
two  processes  trade  places  in  the  morning  hours,  wherein  adenosine  has  been
removed  and  the  rousing  volume  of  the  circadian  rhythm  is  becoming  louder
(indicated  by  the  meeting  of  the  two  lines  in  figure  6),  we  naturally  wake  up
(seven a.m. on day two, in the figure example). Following that full night of sleep,
you  are  now  ready  to  face  another  sixteen  hours  of  wakefulness  with  physical
vigor and sharp brain function.
INDEPENDENCE DAY, AND NIGHT
Have  you  ever  pulled  an  “all-nighter”—forgoing  sleep  and  remaining  awake
throughout the following day? If you have, and can remember much of anything
about it, you may recall that there were times when you felt truly miserable and
sleepy,  yet  there  were  other  moments  when,  despite  having  been  awake  for
longer, you paradoxically felt more alert. Why? I don’t advise anyone to conduct
this self-experiment, but assessing a person’s alertness across twenty-four hours
of total sleep deprivation is one way that scientists can demonstrate that the two
forces determining when you want to be awake and asleep—the twenty-four-hour
circadian  rhythm  and  the  sleepiness  signal  of  adenosine—are  independent,  and
can be decoupled from their normal lockstep.
Let’s consider figure 7, showing the same forty-eight-hour slice of time and the
two  factors  in  question:  the  twenty-four-hour  circadian  rhythm  and  the  sleep
pressure signal of adenosine, and how much distance there is between them. In
this  scenario,  our  volunteer  is  going  to  stay  awake  all  night  and  all  day.  As  the
night  of  sleep  deprivation  marches  forward,  the  sleep  pressure  of  adenosine
(upper line) rises progressively, like the rising water level in a plugged sink when a
faucet has been left on. It will not decline across the night. It cannot, since sleep
is absent.