398 Sleep-Wake Disorders
Development and Course
Shift work type can appear in individuals of any age but is more prevalent in individuals
older than 50 years and typically worsens with the passage of time if the disruptive work
hours persist. Although older adults may sh ow similar rates of circadian phase adjust-
ment to a change in routine as  do younger adults, they appear to experience significantly
more sleep disruption as a consequence of the circadian phase shift.
Risk and Prognostic Factors
Temperamental. Predisposing factors include a morn ing-type disposition, a need for
long (i.e., more than 8 hours) sleep duration s in order to feel well rested, and strong com-
peting social and domestic needs (e.g., parents of young children). Individuals who are able
to commit to a nocturnal lifestyle, with fe w competing day-oriented demands, appear at
lower risk for shift work type. 
Genetic and physiological. Because shift workers are more likely than day workers to
be obese, obstructive sleep apnea may be present and may exacerbate the symptoms.
Diagnostic Markers
A history and sleep diary or actigraphy may be  useful in diagnosis, as discussed earlier for
delayed sleep phase type. 
Functional Consequences  of Shift Work Type 
Individuals with shift work type not only ma y perform poorly at work but also appear to
be at risk for accidents both at work and on the drive home. They may also be at risk for
poor mental health (e.g., alcohol use diso rder, substance use diso rder, depression) and
physical health (e.g., gastrointestinal disorder s, cardiovascular dise ase, diabetes, cancer).
Individuals with a history of bipolar disorder  are particularly vulnerable to shift work
type–related episodes of mania resulting from missed nights of sleep. Shift work type of-