Why We Sleep

Figure 4: The Two Factors Regulating Sleep and Wakefulness

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Now  let  us  consider  what  is  happening  to  the  other  sleep-controlling  factor:
adenosine.  Adenosine  creates  a  pressure  to  sleep,  also  known  as  Process-S.
Represented  by  the  solid  line  in  figure  4,  the  longer  you  are  awake,  the  more
adenosine  builds  up,  creating  an  increasing  urge  (pressure)  to  sleep.  By  mid-  to
late  morning,  you  have  only  been  awake  for  a  handful  of  hours.  As  a  result,
adenosine concentrations have increased only a little. Furthermore, the circadian
rhythm  is  on  its  powerful  upswing  of  alertness.  This  combination  of  strong
activating output from the circadian rhythm together with low levels of adenosine
result in a delightful sensation of being wide awake. (Or at least it should, so long
as your sleep was of good quality and sufficient length the night before. If you feel
as though you could fall asleep easily midmorning, you are very likely not getting
enough  sleep,  or  the  quality  of  your  sleep  is  insufficient.)  The  distance  between
the curved lines above will be a direct reflection of your desire to sleep. The larger
the distance between the two, the greater your sleep desire.
For example, at eleven a.m., after having woken up at eight a.m., there is only a
small  distance  between  the  dotted  line  (circadian  rhythm)  and  solid  line  (sleep
pressure),  illustrated  by  the  vertical  double  arrow  in  figure  5.  This  minimal
difference means there is a weak sleep drive, and a strong urge to be awake and
alert.