392 Sleep-Wake Disorders
Risk and Prognostic Factors
Genetic and physiological. Predisposing factors may include a longer than average cir-
cadian period, changes in light sensitivity, an d impaired homeostatic sleep drive. Some in-
dividuals with delayed sleep phase type may be hypersensi tive to evening light, which
can serve as a delay signal to the circadian cloc k, or they may be hy posensitive to morning
light such that its phase-advancing effects ar e reduced. Genetic factors may play a role in
the pathogenesis of familial and sporadic forms of delayed sleep phase type, including
mutations in circadian genes (e.g., PER3 , CKIe ).
Diagnostic Markers
Confirmation of the diagnosis includes a complete history and use of a sleep diary or actigra-
phy (i.e., a wrist-worn motion detector that mo nitors motor activity for prolonged periods and
can be used as a proxy for sleep-wake patterns for at least 7 days). Th e period covered should
include weekends, when social an d occupational obligations are le ss strict, to ensure that the
individual exhibits a consistently delayed slee p-wake pattern. Biomarkers such as salivary
dim light melatonin onset should be obtain ed only when the diagnosis is unclear.
Functional Consequences of  Delayed Sleep Phase Type
Excessive early day sleepiness is prominent. Ex treme and prolonged difficulty awakening
with morning confusion (i.e., sleep inertia) is also common. The severity of insomnia and
excessive sleepiness symptoms varies substa ntially among individuals and largely de-
pends on the occupational and social demands on the individual. 
Differential Diagnosis
Normative variations in sleep. Delayed sleep phase type must be distinguished from
“normal” sleep patterns in which an individual has a late schedule that does not cause