364 Sleep-Wake Disorders
Aside from the frequency and du ration criteria required to make the diagnosis, addi-
tional criteria are useful to quantify insomnia severity. These quantitative criteria, while
arbitrary, are provided for illustrative purpose only. For instance, difficulty initiating sleep
is defined by a subjective sleep latency greater than 20–30 minutes, and difficulty maintain-
ing sleep is defined by a subjective time awake after sleep onset greater than 20–30 min-
utes. Although there is no standard definition  of early-morning awakening, this symptom
involves awakening at least 30 minutes before the scheduled time and before total sleep
time reaches 6½ hours. It is essential to take into account no t only the final awakening time
but also the bedtime on the previ ous evening. Awakening at 4:00 A.M. does not have the
same clinical significance in those who go to bed at 9:00 P.M. as in those who go to bed at
11:00 P.M. Such a symptom may also reflect an age- dependent decrease in the ability to sus-
tain sleep or an age-dependent shift in the timing of the main sleep period. 
Insomnia disorder involves daytime impairments as well as nighttime sleep difficulties.
These include fatigue or, less commonly, dayt ime sleepiness; the latter is more common
among older individuals and wh en insomnia is comorbid with another medical condition
(e.g., chronic pain) or sleep disorder (e.g., sl eep apnea). Impairment in  cognitive performance
may include difficulties with attention, concentration and memory, and even with performing
simple manual skills. Associated mood disturba nces are typically described as irritability or
mood lability and less commonly as depressive or anxiety symptoms. Not all individuals with
nighttime sleep disturbances are distressed or have functional impairment. For example, sleep