Circadian Rhythms, Sleep Deprivation, and Human Performance - Progress in Molecular Biology and Translational Science - page 164

affecting sleepiness and alertness include the following demand characteris-
tics of the experiment, 61 distractions by environmental stimuli and noise, 62
boredom and motivational factors, 63-65 stress, 66 food intake, 67,68 posture

and activity, 69,70 temperature, 64 lighting conditions, 71,72 ambient and

stimulant drug intake (e.g., caffeine, modafinil, amphetamine). 73-75
Physical, mental, and social activities can represent masking factors that
interact with endogenous circadian rhythms in neurobehavioral functions.
The effects of performing cognitive tests on subjective estimates of alertness
are apparent at certain circadian phases during sleep deprivation. Subjects
report feeling less alert after they are challenged to perform. Thus, prior
activity can influence subjective estimates, and can interact with circadian
effects if not properly controlled when measuring the rhythmicity of subjec-
tive states.
Sleep and sleep loss can also be considered masking factors when mea-
suring circadian rhythmicity in certain neurobehavioral variables. There-
fore, neurobehavioral measures reflect to varying degrees a combination
of endogenous circadian rhythmicity, sleep homeostatic drive, and masking
effects interacting to produce behavioral outcomes.

4. PROTOCOLS TO ASSESS CIRCADIAN VARIATION IN
NEUROBEHAVIORAL FUNCTIONS
Considerable research has been devoted to unmasking circadian
rhythms, that is, eliminating sources of extraneous variance to expose the
endogenous circadian rhythms of variables of interest, including alertness
and cognitive performance. Two such experimental approaches are the
use of a constant routine protocol and the use of a forced desynchrony
protocol.

4.1. Constant routine
The constant routine procedure 76 is generally regarded as the gold standard
for measuring circadian rhythmicity. By keeping subjects awake with a fixed
posture in a constant laboratory environment for at least 24 h, circadian
rhythms in a variety of physiologic and neurobehavioral variables can be
recorded without biases ( Fig. 7.1 ). Indeed, the circadian rhythm of body
temperature is believed to be largely free of masking effects when derived
under a constant routine.
However, when neurobehavioral variables are considered, sleep depri-
vation and the stimuli used to sustain wakefulness can constitute masking