Why We Sleep

Figure 5: The Urge to Be Awake

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However, by eleven p.m. it’s a very different situation, as illustrated in figure 6.
You’ve  now  been  awake  for  fifteen  hours  and  your  brain  is  drenched  in  high
concentrations  of  adenosine  (note  how  the  solid  line  in  the  figure  has  risen
sharply).  In  addition,  the  dotted  line  of  the  circadian  rhythm  is  descending,
powering  down  your  activity  and  alertness  levels.  As  a  result,  the  difference
between the two lines has grown large, reflected in the long vertical double arrow
in  figure  6.  This  powerful  combination  of  abundant  adenosine  (high  sleep
pressure)  and  declining  circadian  rhythm  (lowered  activity  levels)  triggers  a
strong desire for sleep.