Short power naps energise but don't make up for a long sleep.

Ever woken up from an afternoon nap and felt groggy? If the answer is yes, Adelaide University’s sleep expert Fiona Kerr believes you have slept too long in the “no-go zone”.

There are magic amounts of time perfect for napping which range from power naps to longer overnight sleeping patterns which allow your brain to conduct maintenance.

Dr Kerr said many little naps throughout the day did not make up for a single longer sleep.

“What happens in your long sleep is the first few cycles, so for the first five to six hours your brain cleans,” she told 891 ABC Adelaide.

She said cleaning includes taking sticky plaque off neutrons which helps to prevent Alzheimer’s.

By about the sixth hour of sleep the human brain starts to file information and what it learnt during the day.

“The way our memory works, we take the information into a short-term memory bank and we sit it there and then we see if we go back for it or not,” Dr Kerr said.

After that work is complete the brain begins to think creatively, because the frontal lobe powers down during REM sleep.

“It takes information and combines and re-combines and takes information out of files that weren’t connected and reconnects them.

“So that’s why you get that wonderful buzz when you wake up, and you get different ideas, because you are combining things that aren’t supposed to be.”

Napping ‘valley of death’

People tend to skip the creative ideas section of sleep if they are not getting enough of a long sleep.

The sweet spot for one nap a day is between 20 and 30 minutes to “clean your inbox” of short-term memories.

She said after a short nap people often felt fresher, had increased memory, alertness, mood and motor skills.

But she warned when you kept going beyond that, sleeping between half an hour and an hour, you entered the napping “valley of death”.

“You either go before half an hour where you have a short sharp nap, and you are up and alert and off you go, or you go for the long haul between one and one-and-a-half hours,” she said.

“What happens, because you have already had the maintenance at night, your brain can more quickly go into the REM.”

Often people describe being “fuzzy headed” after an hour-long sleep because the frontal lobe had powered down and needed time to re-energise.

This post originally appeared on ABC News.