By Jim Leffman via SWNS
A short afternoon siesta can boost memory, mood and alertness for at least four hours after waking, a new study reveals.
Researchers found the optimum time was 30 minutes with a further 10 minute window to nod off, though anything up to an hour had a positive effect.
The team from the National University of Singapore defined a nap as a voluntary sleep as opposed to uncontrollable or unintended shut-eye.
They said this could be a sign of significantly inadequate night time sleep, jetlag, a sleep disorder, or a neurological condition.
Dr. Ruth Leong, research fellow from the Centre for Sleep and Cognition at the Yong Loo Lin School of Medicine at the university said: "Many know the benefits of napping, but the pressure to optimize time in the workday poses constraints for some on the practicality of napping regularly.
"We had to ask is there a recommended duration for a mid-afternoon nap that achieves a balance between practicability and meaningful benefits?"
Following their usual amount of sleep at night, 32 young adults underwent four experimental conditions: wake, a 10-minutes nap, a 30-minutes nap, and a 60-minutes nap on separate days.
Mood, subjective sleepiness, cognitive performance were measured at intervals of five, 30, 60 and 240 minutes after waking in order to compare how sustained the respective nap benefits were.
The effect of these nap lengths on memory encoding was also examined.
Compared to staying awake, all nap durations ranging from 10 to 60 minutes had clear benefits.
A boost for positive mood, subjective sleepiness, and alertness lasted up to 240 minutes post-nap, suggesting that even a short 10-minutes can act as a refresh.
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However only the 30-minutes nap had benefits for memory encoding, indicating that at least 30 minutes may be needed for benefits to memory.
The downside was that sleep inertia, a groggy feeling just after waking that can result in a temporary dip in performance was only observed for the 30 to 60 minutes naps.
However these effects had disappeared within 30 minutes of waking.
Overall, improvements for vigilance were moderate and benefits for speed of processing were not seen.
Leong added: "While no clear ‘winning’ nap duration was found, a 30-minutes nap appears to have the best trade-off between practicability and benefit.
"So 30 minutes may be the duration to recommend for a mid-afternoon nap, while provisioning an additional ~10 minutes to fall asleep."
The study was published in the journal SLEEP.