Sleep disorders

Aetiology

More is known about the causes of the dyssomnias than the parasomnias.

Dyssomnias

Insomnia
The common causes of transient and chronic insomnia are shown in the table.

Hypersomnia
As with chronic insomnia, both sleep apnoea and narcolepsy can be caused by psychiatric disorders, eg depression and other medical diseases, eg cardiac problems. Both these types of hypersomnia are also believed to be have a genetic component to their aetiology. Specific causes of sleep apnoea include obesity and snoring, while specific causes of narcolepsy are not well defined, although reduced brain concentrations of dopamine and noradrenaline have been found in people with the disorder. Narcolepsy shows a strong relationship with the HLA subtypes. However, this is probably related to a genetic defect, as correlations have been found with specific chromosome subtypes.(1,4)

Circadian rhythm  sleep disorders 
Jet lag is caused by flying across time zones. Shift work sleep disorder sleep disorder is caused by working shifts, especially nights. Delayed sleep phase disorder (DSPS) may be caused by behaviour, eg going to bed later than the desired time, or by physiological abnormalities in the body's circadian clock. Similarly, advanced sleep phase disorder (ASPS) may be caused by age-related changes in the circadian clock.

Parasomnias
Little is known about the causes of sleepwalking and sleep terror, but the latter is believed to have a genetic component. Parasomnias can occur in people who have post-traumatic stress disorder.

What Happens in the Brain? 

Sleep and wakefulness are understood to be generated by the activation of specific neural centres in the brain. One of the most important centres is the reticular activating system (RAS), which is located in the brainstem  region. A key component of the RAS is a group of cholinergic nuclei near the pons -midbrain junction. Many of the neurons  in these nuclei are highly active during waking and REM sleep, and inactive during non-REM sleep.

As well as the acetylcholine released from these neurons, several other neurotransmitters  from the brainstem are also involved in the regulation of the sleep and wake states. These include noradrenaline, serotonin, histamine, glutamate and vasopressin.

Some types of sleep disorders may arise from abnormalities or changes in the areas of the brain and the neurotransmitters involved in the control of sleep.

In addition, some sleep disorders may be associated with abnormalities in specific stages of sleep, for example people with narcolepsy have a shortened sleep onset REM (SOREM).(2,3)

References

1. Billiard M, Seignalet J, Besset A, Cadilhac J. HLA-DR2 and narcolepsy. Sleep 1986;9(1 Pt 2):149-52.

2. Billiard M, Salva MQ, De Koninck J, et al. Daytime sleep characteristics and their relationships with night sleep in the narcoleptic patient. Sleep 1986;9(1 Pt 2):167-74.

3. Nakayama J, Miura M, Honda M, et al. Linkage of human narcolepsy with HLA association to chromosome 4p13-q21. Genomics 2000;65:84-6.

4. Bixler EO, Kales A, Vela-Bueno A, et al. Narcolepsy/cataplexy III: Nocturnal sleep and wakefulness patterns. Int J Neurosci 1986;29:305-16