Circadian Rhythms, Sleep Deprivation, and Human Performance - Progress in Molecular Biology and Translational Science - page 165

factors. In constant routine experiments, these masking effects are evident in
subjective measures of sleepiness and alertness. 57,77 Figure 7.1 shows the
somewhat reduced values for subjective alertness as well as cognitive and
psychomotor performance after 30 h awake in a constant routine, compared
with the values of these variables 24 h earlier (i.e., at the same circadian phase
but without sleep deprivation).
Recently, the constant routine protocol has been used to examine
metabolites in saliva and plasma at different times of day to identify those
that are under circadian control and are independent of sleep. 78,79 Remark-
ably, one study found that metabolites from blood taken every 2 h, which
were used to form a circadian timetable, could subsequently be used to pre-
dict internal time within a 3-h interval using only two blood samples. 80
More recently, a constant routine was used to examine the effects of chronic
sleep restriction on circadian rhythmicity and amplitude of genes that were
upregulated or downregulated using a transcriptome analysis, highlighting
the critical interaction between sleep homeostasis and circadian rhythms
at the mRNA level. 81
A progressive change associated with the time spent awake is typically
superimposed on the circadian rhythm of neurobehavioral variables. 82,83
When total sleep deprivation is continued for several days (whether in a con-
stant routine procedure or an experimental design involving ambulation),
the detrimental effects on alertness and performance increase, and although
the circadian process can be exposed, 84 it is overlaid on a continuing (nearly
linear) change reflecting increasing homeostatic pressure for sleep. 85 This is
illustrated in Fig. 7.2 for PVT performance lapses—perhaps the most sensi-
tive waking measure of homeostatic sleep drive and circadian phase, and the
least masked by aptitude and learning. 25,86 It is noteworthy that decreased
alertness during the circadian trough is associated with increased intra-
individual variability in performance. This is evidenced by intermittent laps-
ing (reaction times > 500 ms) 87 which reflects wake state instability. 24,25 The
wake state instability hypothesis posits that sleep-initiating mechanisms may
interfere with wakefulness, making sustained performance unstable and
dependent on compensatory mechanisms. 25

4.2. Forced desynchrony
The forced desynchrony protocol 88,89 conducted in temporally and envi-
ronmentally isolated conditions, is an experimental procedure particularly
suitable for studying the interaction of the circadian and homeostatic pro-
cesses. 55,90,91

In this protocol, a subject's imposed timing and duration of