<i>How long is the ideal nap?</i>

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?
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%.
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 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.
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.
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.
The timing of the nap is also important. Early afternoon is often the best time, between 1-3pm, she says, when people experience a post-lunch dip in energy.