Why We Sleep

Figure 3: Effects of Various Drugs on Spider Web Building
It is worth pointing out that caffeine is a stimulant drug. Caffeine is also the
only  addictive  substance  that  we  readily  give  to  our  children  and  teens—the
consequences of which we will return to later in the book.
IN STEP, OUT OF STEP
Setting  caffeine  aside  for  a  moment,  you  may  have  assumed  that  the  two
governing forces that regulate your sleep—the twenty-four-hour circadian rhythm
of  the  suprachiasmatic  nucleus  and  the  sleep-pressure  signal  of  adenosine—
communicate with each other so as to unite their influences. In actual fact, they
don’t. They are two distinct and separate systems that are ignorant of each other.
They are not coupled; though, they are usually aligned.
Figure 4
 encompasses  forty-eight  hours  of  time  from  left  to  right—two  days
and two nights. The dotted line in the figure is the circadian rhythm, also known
as Process-C. Like a sine wave, it reliably and repeatedly rises and falls, and then
rises  and  falls  once  more.  Starting  on  the  far  left  of  the  figure,  the  circadian
rhythm begins to increase its activity a few hours before you wake up. It infuses
the  brain  and  body  with  an  alerting  energy  signal.  Think  of  it  like  a  rousing
marching  band  approaching  from  a  distance.  At  first,  the  signal  is  faint,  but
gradually  it  builds,  and  builds,  and  builds  with  time.  By  early  afternoon  in  most
healthy adults, the activating signal from the circadian rhythm peaks.