Circadian Rhythms, Sleep Deprivation, and Human Performance - Progress in Molecular Biology and Translational Science - page 157

considered more pressing or of greater value. 13 Analyses of the American
Time Use Survey (ATUS) revealed that paid work time and commuting
to and from work were the two waking activities most often exchanged
for sleep time. 14 Sleep time was lowest in the 45- to 54-year-old respon-
dents, shorter in men than in women, and shorter on weekdays compared
to weekends. An ATUS analysis on waking activities in the 2-h period
before retiring in the evening and after waking up in the morning showed
that watching TV was the dominant ( > 50%) activity in the 2 h before retir-
ing. 15 Long work hours were associated with progressively earlier wake-up
times in the morning, while long-hour workers, short-hour workers, and
those who did not work did not differ in the times when they retired at
night. 15 We speculate that some of this sleep-restriction behavior may be
explained by respondents with a late evening circadian phase preference,
who awaken early by alarm clock to commute for paid work. These indi-
viduals cannot easily advance their sleep onset, but they can use an alarm
clock to advance their sleep offset (for commuting and paid work), resulting
in a restricted sleep period. This misalignment of biological and social time
has been termed “social jet lag” by Roenneberg and colleagues. 16 Individ-
uals with a late circadian preference thus often engage in chronic sleep
restriction during the work week, and try to pay off their sleep debt on
the weekend. Furthermore, shift work affects sleep and alertness of approx-
imately one out of five working Americans, with 15% of full-time salaried
workers usually working shifts that include nights. 17 Shift work includes
working evenings, nights, or rotating shifts and is often associated with
shorter-than-normal and disrupted sleep periods at an adverse circadian
phase. 18 The International Agency for Research on Cancer concluded in
2007 that shift work involving circadian disruption is probably carcinogenic
to humans. 17,19
2. SLEEP - WAKE AND CIRCADIAN REGULATION:
TWO-PROCESS MODEL
The two-process model of sleep-wake regulation has been applied to
the temporal profiles of sleep 20,21 and daytime vigilance. 22 The model con-
sists of a homeostatic process (S) and a circadian process (C), which combine
to determine the timing of sleep onset and offset. The homeostatic process
represents the drive for sleep that increases as a saturating exponential during
wakefulness (as can be observed when wakefulness is maintained beyond
habitual bedtime into the night and subsequent day) and decreases as a