BioEd Online

You might think it's obvious when someone is sleepy: drooping eyelids, a wide yawn and a nodding head tend to give it away. But researchers have added another layer of precision to the picture, by finding an enzyme that increases with sleepiness.

The discovery could help to unpick some of the biological mechanisms behind sleep, and represent a novel way to develop on-the-spot tiredness tests for drivers, pilots and doctors working long or irregular hours.

It has proven challenging to find accessible markers of sleepiness most current efforts involve using overt signs, such as fluttering eyelids. But 'sleepy' behaviour varies a lot from person to person.

To come up with a more definitive marker, Paul Shaw at Washington University School of Medicine in St Louis, Missouri, and his colleagues studied flies subjected to sleep deprivation. They found that levels of an enzyme called amylase, which is involved in breaking down starch, gets higher and higher the longer the flies are awake.

When they tested sleep-deprived humans, levels of amylase in saliva were also higher the longer the volunteers went without sleep. It isn't clear why this is: amylase isn't known to have any sleep-related functions, although it is related to stress.

Rest of the picture

The amount of variation in amylase from person to person means that a single measurement can't, for now, tell whether an individual is tired. But, says Shaw, relative levels of amylase against other compounds in saliva, along with other biomarkers, might reveal something more telling. "We think that ultimately we're going to need a panel of biomarkers," he says. They're on the hunt for these now. "More markers give you more sensitivity and reliability," he adds.

So will sleepiness soon be subject to testing? "It would be marvellous if we could have an instantaneous test," says Jim Horne, director of the Sleep Research Centre at Loughborough University, UK. But he worries that such a test wouldn't be very reliable. "If people want to beat the system they always can."

But the search for biomarkers might have other payoffs. Shaw hopes that he will come across other markers that have a more direct role in sleep, which could shed light on the biology of snoozing and help insomniacs, for example, achieve sleep. "I think that as we start looking for more biomarkers, we'll find some markers that are more causally involved. Then those become targets for treatment and therapeutics."

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