Circadian Rhythms, Sleep Deprivation, and Human Performance - Progress in Molecular Biology and Translational Science - page 167

fibroblasts from skin biopsies or blood samples. 100-102 While these in vitro
skin fibroblasts can determine circadian rhythms and period, they do not
necessarily correlate with in vivo physiological rhythms, 97 limiting the validity
and utility of this technique. Standard physiological estimates of circadian phase
include the dim light melatonin onset 103 and core body temperature mini-
mum. 57,58 These methods are important for characterizing interindividual
variation in circadian rhythmicity.
5.1. Chronotype (morningness-eveningness)
Chronotype or morningness-eveningness (i.e., the tendency to be an early
“lark” or a late “owl”) is perhaps the most frequently measured inter-
individual variation in circadian rhythmicity. Morning- and evening-type
individuals differ endogenously in the circadian phase of their biological
clocks. 57,58 Self-report questionnaires, such as the Horne- ¨ stberg
morningness-eveningness questionnaire 104 and its variants, 105 and the
Munich ChronoType Questionnaire, 106,107 differentiate timing of activities
on workdays versus free days. They are the most commonly utilized mea-
sures of circadian phase preference, because of their convenience and cost
effectiveness.
Age affects morningness-eveningness as shown in laboratory studie s 108
and more naturalistic population-based settings. 107,109 In addition, gender
influences morningness-eveningness with women showing a greater skew
toward morningness than men. 107,110-112 Women also have been reported
to have a shorter average intrinsic circadian period than men, 113 though
not consistently, 114 and blacks have been reported to have a shorter
average intrinsic circadian period than whites. 114 These differences in
circadian phase preference (and possibly in circadian period) appear to be
enduring traits, with a significant genetic basis across various diverse
populations. 115-120 As such, chronotype is a phenotypic aspect of circadian
rhythmicity in humans. 121
In line with the two-process model, the relationship of chronotype to the
regulation of sleep and neurobehavioral responses to sleep deprivation has
been investigated in laboratory studies. Chronotypes showed differences
in homeostatic sleep regulation 122-124 and in homeostatic response to
sleep fragmentation. 125 Moreover, chronotypes showed differences in
neurobehavioral responses to sleep fragmentation 126 and to total sleep
deprivation, 127 and to risk-taking propensity at baseline and following
total sleep deprivation. 128