366 Sleep-Wake Disorders
Risk and Prognostic Factors 
While the risk and prognostic factors discussed in this section increase vulnerability to in-
somnia, sleep disturbances are more likely to  occur when predisposed individuals are ex-
posed to precipitating events, such as major lif e events (e.g., illness, separation) or less
severe but more chronic daily stress. Most individuals resume normal sleep patterns after
the initial triggering event ha s disappeared, but others—perha ps those more vulnerable to
insomnia—continue experiencing persistent sleep difficulties. Perpetuating factors such as
poor sleep habits, irregular sleep scheduling, an d the fear of not sleeping feed into the in-
somnia problem and may contribute to a vicious  cycle that may induce persistent insomnia.
Temperamental. Anxiety or worry-prone personality or cognitive styles, increased arousal
predisposition, and tendency to repress emot ions can increase vulne rability to insomnia.
Environmental. Noise, light, uncomfortably high or low temperature, and high altitude
may also increase vulnerability to insomnia.
Genetic and physiological. Female gender and advancing age are associated with in-
creased vulnerability to insomn ia. Disrupted sleep and insomnia display a familial dispo-
sition. The prevalence of insomnia is hi gher among monozygoti c twins relative to
dizygotic twins; it is also higher in first-degree family members comp ared with the general
population. The extent to which this link is  inherited through a genetic predisposition,
learned by observations of pare ntal models, or established as  a by-product of another psy-
chopathology remains undetermined.
Course modifiers. Deleterious course modifiers incl ude poor sleep hygiene practices
(e.g., excessive caffeine use,  irregular sleep schedules).
Gender-Related Diagnostic Issues