**3.3 Irregular sleep-wake rhythm disorder (ISWRD)**

ICSD-3 Diagnostic Criteria [6]:


Clinical Characteristics: ISWRD is characterized by a temporally disorganized sleep and wake pattern such that there are no clearly defined periods of wake and/ or sleep. Multiple sleep and wake periods can occur throughout the day, and usually consist of 3 or more short intervals, approximately 1-4 hours each. The longest period generally occurs in the morning between 2 and 6 AM; however, the distribution of sleep and wake periods can vary per individual. The total sleep duration throughout a 24-hour period is generally normal for the individual's age. Because of the fragmented nature of sleep, individuals can experience frequent napping, excessive daytime sleepiness, and difficulty staying asleep at night.

#### **3.4 Non-24 sleep-wake rhythm disorder (N24SWRD)**

ICSD-3 Diagnostic Criteria [6]:


#### *Circadian Rhythm Disorders DOI: http://dx.doi.org/10.5772/intechopen.99816*

Clinical Characteristics: N24SWD, also known as non-trained rhythm disorder, is characterized by chronic cycles of sleep and wake that are not always synchronized with the 24-hour environment. There is a gradual, but consistent drift of sleep and wake times later into the day. Attempting to maintain a regular sleep–wake schedule can lead to symptoms of excessive daytime sleepiness, chronic fatigue, early morning awakenings and insomnia. These symptoms alternate with days to weeks in which the patient is asymptomatic, owing to the patient's endogenous circadian system coinciding with the external 24-hour cycle. Napping is common, and patients often report impairment of social and occupational functioning due to non-entrained sleep–wake schedule.

N24SWD occurs most often in blind individuals. Onset of symptoms usually occurs in 2nd or 3rd decade of life, and men are disproportionally affected compared to women at a ratio of >2: 1 [7].
