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Studies of people with narcolepsy are changing the way we understand the influence of chemicals called orexins on sleep, appetite and much more.

Narcolepsy is a disorder that causes excessive daytime sleepiness, often with cataplexy (sudden muscle weakness caused by strong emotions) and disruptions to night-time sleep.

There’s evidence that people with narcolepsy exhibit different eating behaviours to the general population, and that these go beyond the effects of just being tired. Understanding more about this is important because obesity is twice as common in people with narcolepsy, and narcolepsy has also been linked to eating disorders.

Snacking for science

One study published in 2016 showed how people with narcolepsy respond differently to food. The research involved computerised tasks where participants could win salty or sweet snacks. Usually, when someone eats all they want of, say, the sweet snacks, these become ‘devalued’ in their eyes and they make fewer responses to win more in subsequent tasks. However, people with narcolepsy didn’t show this adjustment.

What’s more, when allowed to eat freely from bowls of snacks after the task, people with narcolepsy consumed almost four times as many calories as their healthy counterparts.

Finding links between narcolepsy and appetite isn’t that surprising. Narcolepsy is caused by the loss of brain cells that make two chemicals called orexins (also known as hypocretins). Their effects are far-reaching: as well as being involved in the sleep–wake cycle, they influence motivation, play a part in appetite, help control the cardiovascular and hormonal systems and are involved in heat regulation.

The effects on appetite observed seem to go beyond those of just not getting a good night’s sleep. The researchers concluded: “These results support our interpretation that orexin deficiency in particular is associated with a diminished coupling between subjective experiences and food choices.”

Nocturnal happenings

Researchers have identified other unusual behaviours that are more common in narcolepsy, some of which could be contributing to the higher rate of obesity. These include sleep-related eating disorder, in which people involuntarily eat or drink during the night. This can cause dental problems and is dangerous: people risk injuries from cooking when not completely alert, starting fires, choking and even ingesting poisonous things.

There’s also nocturnal smoking, an “out of control” behaviour where people feel compelled to smoke when they wake at night.

A 2011 study found that 32 per cent of people with narcolepsy questioned had a sleep-related eating disorder, compared to 3 per cent of people in a group matched by age, sex and where they live. Nocturnal smoking was reported by 21 per cent of people with narcolepsy but was absent from the non-narcolepsy group.

The researchers concluded that the disorders were linked to a lack of orexins, and that interactions between this chemical system and another in the brain, based on dopamine, were likely behind these losses of control.

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