RT Book, Section
A1 Pack, Allan I.
A2 Grippi, Michael A.
A2 Antin-Ozerkis, Danielle E.
A2 Dela Cruz, Charles S.
A2 Kotloff, Robert M.
A2 Kotton, Camille Nelson
A2 Pack, Allan I.
SR Print(0)
ID 1194946119
T1 Sleep and Circadian Rhythms
T2 Fishman’s Pulmonary Diseases and Disorders, 6e
YR 2023
FD 2023
PB McGraw-Hill Education
PP New York, NY
SN 9781260473988
LK accessmedicine.mhmedical.com/content.aspx?aid=1194946119
RD 2025/03/18
AB Sleep  and  circadian  rhythm  are  highly  coupled  processes.  In  the  original  formulation  they  were  considered  independent  but  interacting.  Borbely  and  colleagues1–4  posited  that  the  circadian  process  (Process  C)  had  a  24-hour  rhythm  that  interacted  with  the  sleep  drive  system  (Process  S)  (Fig.  12-1).  Process  S  is  envisaged  to  be  like  an  old-fashioned  egg  timer.  The  drive  for  sleep  is  at  a  very  low  level  following  the  major  sleep  bout  and  increases  progressively  as  wakefulness  proceeds,  i.e.,  the  drive  to  sleep  is  related  to  the  duration  of  prior  wakefulness.  Humans  are  programmed  to  sustain  wakefulness  for  16  hours  but  beyond  this  develop  progressive  performance  impairments.  During  the  day  the  drive  to  sleep  is  counteracted  by  an  alertness  signal  from  the  clock.  When  this  alertness  signal  declines  later  in  the  evening,  the  sleep  drive  is  unopposed,  and  sleep  ensues.  During  sleep  the  drive  to  sleep  progressively  declines,  i.e.,  the  egg  timer  is  flipped,  and  the  sands  recover  (Fig.  12-1).  The  situation  is  actually  more  complex  than  this  since  sleepiness  occurs  at  two  times  of  day,  i.e.,  siesta  time  in  early  afternoon  and  late  in  the  evening.  While  these  processes  were  initially  considered  independent,  they  are  not  at  a  molecular  level.5,6  Core  clock  molecules  increase  their  expression  in  brain  when  sleep  is  deprived.7  Moreover,  mutations  of  a  clock-associated  gene—DEC2,  now  called  BHLEH41—result  in  short  sleep  in  humans  (<6  hours)  without  evidence  of  daytime  performance  impairment.8,9