Image caption Not the best napping technique

Air traffic controllers in the US have been advised to take 26-minute naps, after a string of incidents involving workers falling asleep. So is 26 minutes the ideal length of time for a nap?

Five cases of air traffic controllers falling asleep on the job have been revealed since March.

In three of those cases, disclosed by the Federal Aviation Association, workers have been fired.

Now the National Transportation Safety Board (NTSB) is calling for "controlled naps" to be built into night shifts.

Referring to a 1995 study from Nasa, which he co-authored, NTSB member and fatigue expert Mark Rosekind said that a 26-minute nap would improve performance by 34% and alertness by 54%.

The answer Experts differ on the ideal length of a nap

Some say between 20 and 30 minutes

Others believe napping beyond 20 minutes risks you feeling groggy afterwards

Drinking a cup of coffee before the nap can help you wake in about 20 minutes

There was other supporting evidence that said naps of between 20 minutes and 30 minutes were beneficial, he said.

His call for work naps is supported by the controllers' union, which wants naps to be allowed in both overnight and day shifts.

Beyond the aviation industry, combating fatigue is an issue that affects many people across all professions, working day and night, although it carries obvious risks in jobs that involve motoring or machinery.

But other experts are doubtful that 26 minutes is the optimum napping time.

It's a bit too long and risks you falling into a deep sleep, says Jim Horne, director of the Sleep Research Council in the UK, which advises the government on guidelines for drivers.

Once you get beyond 20 minutes, you risk a deep sleep and you can be much more groggy when you wake up Jim Horne, Sleep Research Council, UK

"What we recommend is that a nap is combined with a cup of coffee so you have some caffeine, and that takes about 20 minutes to kick in.

"Have a cup of coffee and get your head down. Done together it has a more powerful effect."

It probably works out that a nap of about 15 minutes is best, he says, because once you get beyond 20 minutes, you risk a deep sleep and you can be much more groggy when you wake up.

"A lot of people take caffeine after they wake up, but you have a window of opportunity of 20 minutes, so it will help you wake up. It works, there's no doubt about it."

People can't instantly fall asleep, so it's impossible to exactly time how long you will be asleep, he says. But even 15 minutes of dozing is beneficial.

"At least by having caffeine, you know that in 20 minutes you will feel more alert."

If you haven't had a wink of sleep the night before, then this tactic won't be enough to refresh you, says Mr Horne, but for those that have had merely a poor night's sleep, it will work.

Early or late?

Longer naps would work if they became part of your daily routine, he says, because your body would get used to it and could wake up quite easily without feeling too groggy.

Longer naps can work too A 20-minute snooze can enhance alertness

Limit the nap to 45 minutes if you need to spring into action on waking

A 60-minute nap improves alertness for 10 hours

Naps of 90-120 minutes encompass all stages of sleep and help clear the mind Jennifer Ackerman, author of Sex Sleep Eat Drink Dream: A Day in the Life of Your Body

Health writer Linda Wasmer Andrews, based in Albuquerque, New Mexico, also believes 26 minutes is too long. She says a nap of between 10 and 20 minutes is enough.

The timing of the nap is also important, she says. Putting your head down too early means your body may not be ready to sleep yet, but a nap that is too late in the day might make it harder to fall asleep come bedtime.

Early afternoon is often the best time, between 1-3pm, she says, when people experience a post-lunch dip in energy.

Whatever the best strategy is, it's unlikely that the US air traffic controllers will be adopting any such tactics soon.

Transport Secretary Ray LaHood has dismissed the proposal for on-the-job naps to be implemented in the aviation industry.

He said workers would not be paid to sleep, and instead ordered for more managers be hired to supervise nightshift workers and ensure they don't fall asleep on the job.