It explains about 5 to 7 per cent of cases of hypersomnia seen in sleep disorders centres. Women are more susceptible than men.

Excessive daytime sleepiness is reported. Subjects show an elevated score on the Epworth sleepiness scale. Night sleep is perceived as non-restorative and generally of poor quality. Patients are often intensely focused on their hypersomnia, and psychiatric symptoms typically become apparent only after prolonged interview or psychometric testing. Poor work attendance, abruptly leaving work because of a perceived need to sleep are common. Polysomnography typically shows a prolonged sleep latency, an increased wake time after sleep onset, and a low sleep efficiency. REM latency may be shortened in the case of bipolar disorder. Contrasting with the elevated score on the Epworth sleepiness scale, sleep latency on the MSLT is often within normal limits. A 24 h continuous sleep recording typically shows considerable time spent in bed during day and night, a behaviour referred to as clinophilia, from the Greek κλινη (bed) and φιλεω (love).

Psychiatric interview is essential to diagnose the underlying condition. Causative psychiatric conditions include bipolar type II disorder, dysthymic disorder, undifferentiated somatoform disorder, adjustment disorder, or personality disorder.

Conventional drugs such as antidepressants or anxiolytics are often insufficient. Modafinil, an awakening drug given at a daily dose of 100 to 200 mg, is usually active.

In the group of psychiatric disorders a separate place should be reserved to seasonal affective disorder remarkable for episodes of major depression occurring only during the winter months, associated with fatigue, loss of concentration, increased appetite for carbohydrates, weight gain, and increased sleep duration. Morning bright light treatment (2500 lux for 2 h) is efficient.

A wide spectrum of medications used in psychiatry may be responsible for excessive sleepiness.

According to ICSD-2, this syndrome occurs when an individual persistently fails to obtain the amount of sleep required to maintain normal levels of alertness. Behaviourally induced insufficient sleep syndrome is likely the most common cause of daytime sleepiness. In a population-based study conducted in Japan among 3030 subjects aged 20 years and older, 29 per cent slept less than 6 h per night, and 23 per cent reported having insufficient sleep.⁽³⁾ The syndrome is likely to be widespread in truck drivers, working mothers, family doctors, executives, and students. The main symptoms are excessive sleepiness in the afternoon or early evening, decrease of diurnal performances, and, of interest to the psychiatrist, irritability, nervousness, and depression. Diagnosis of the syndrome is relatively easy provided that a thorough interview is conducted. The most rational treatment is an increase of daily total sleep time, either by spending more time in bed at night, or by taking one or two naps per day.

The most frequent condition among these disorders is the obstructive sleep apnoea syndrome. This syndrome was first described by
Guilleminault et al.⁽⁴⁾ It is most frequent in 50-year-old males. According to Young et al.⁽⁵⁾ the prevalence of obstructive sleep apnoeas accompanied by excessive daytime sleepiness in North America is 4 per cent in men and 2 per cent in women.

Clinical features include night-time and daytime symptoms. Night-time symptoms are represented by loud snoring, apnoeic episodes ending with sonorous breathing resumption, nocturia, severe fatigue upon awakening, and sometimes headache. Daytime symptoms are dominated by excessive sleepiness, which varies in intensity among patients. Other symptoms include irritability, negligence, loss of concentration, loss of libido, impotence, and sometimes depression.

Patients are often obese or mildly obese. High blood pressure is a frequent feature. The ear, nose, and throat examination usually reveals a narrow upper airway due to close-set posterior tonsillar pillars, an abnormally long and hypotonic soft palate, a hypertrophic uvula, or macroglossia.

The positive diagnosis rests on polysomnography allowing the observation of nocturnal disrupted sleep and the identification of apnoeas and of their type (obstructive, central, or mixed) as well as their consequences on heart rate, oxygen desaturation, and degree of somnolence.