A review of studies has found that, contrary to expectations, evidence supporting early brain ageing does not appear consistently in narcolepsy patients who suffer severe sleep disorders. The observation is unusual given existing research linking sleep disorders to cognitive decline and dementia.

Japan's Gigazine reported on Oct. 15 that a research team centred on Brazil's Federal University of Sao Paulo compiled previous studies on the link between type 1 narcolepsy and brain ageing in a paper published in the journal Ageing Research Reviews.

Narcolepsy is a neurological disorder in which problems in regulating sleep and wakefulness cause daytime sleepiness that is hard to endure. Patients may also wake frequently at night or experience sleep paralysis or hallucinations while falling asleep. Type 1 is characterised by cataplexy, a sudden loss of muscle strength triggered by emotions such as laughter or surprise, and involves a deficiency of the neuropeptide orexin (hypocretin), which helps maintain wakefulness.

Persistent sleep disorders are generally linked to cognitive decline and a higher risk of dementia. For that reason, type 1 narcolepsy was presumed to involve faster brain ageing, but existing biomarker and brain imaging studies have not consistently confirmed it.

The researchers focused on the possibility that orexin deficiency could explain the discrepancy. Orexin is involved not only in wakefulness but also in inflammatory responses, neuronal activity and the processing of amyloid beta. Animal experiments have reported that suppressing orexin signalling reduced the accumulation of amyloid beta, a protein associated with Alzheimer's disease. Changes in sleep time may also have affected the result.

A 2008 animal study found that mice lacking orexin and normal mice had similar total wake time, but differed in their ability to stay awake for long periods.

Narcolepsy patients do not necessarily sleep longer. Even if they take naps, frequent interruptions to night sleep may mean total sleep time does not increase much. The researchers stressed that "it does not mean the disease prevents dementia" and said "we need to examine not only the amount of sleep but also how wakefulness is sustained and the role of orexin".