Opposite to intensity of exercise, Rudzik and Ball (Chap.3, this volume)
examined subjective and objective measures of sleep quality and fatigue. For
example, subjective data drawn from sleep diaries can be used to check or resolve
anomalies in objective data drawn from actigraph monitors. Similarly, sleepiness
can be measured by subjective instruments as well as objective means. The
pupillographic sleepiness test (PST) measuresfluctuations in pupil size in the dark,
which reflects central nervous system activation and thus alertness (Eggert et al.
2012 ; Wilhelm et al. 2015 ). Subjective measures of sleepiness show lower repro-
ducibility than the PST, perhaps because of factors such as motivation, expectation,
and mood (Wilhelm et al. 2015 ).
From a broader perspective, Agogo et al. ( 2014 ) grapple with the various ways
in which aging can be measured and perceived. Chronological age represents time
lived on earth; however, the aging process is not regulated by chronological time.
Biological aging, or senescence, has been quantified in a variety of ways, including
blood pressure (cardiovascular system), peak expiratoryflow (respiratory system),
grip strength (muscle mass and strength), waist circumference (metabolic dysreg-
ulation), and telomere length (cellular aging) (Stephan et al. 2015 ). Mental aging,
according to Agogo et al. ( 2014 ), refers to changes in an individual’s cognitive
capability over time, including perception, memory, and executive function. Social
aging refers to the phases of the human life course in relation to political, social,
economic, cultural, and demographic factors. Subjective age (or identity age) refers
to how an individual perceives himself or herself in relation to a reference
age-group. Figure15.4illustrates the relationship between subjective and chrono-
logical age, and how that relationship can shift or“bend.”For example, unhealthy
eating and smoking can increase subjective biological age, while physical activity
can decrease subjective biological age. The cessation of reading can increase
Fig. 15.4 Subjective age and chronological age model (Agogo et al. 2014 : 388)
15 The Shrinking Black Box of Human Biology 319