Dolphins, known to be intelligent, have little or no REM sleep

Dr Robert Stickgold of Harvard University told BBC Radio 4's Today programme dreams edited events people had experienced.

Learning ability

Professor Siegal's research looked at the belief that REM sleep helped consolidate memories.

Professor Siegal, head of neurobiology research at the VA Greater Los Angeles Healthcare System Sepulveda, California, and professor of psychiatry at UCLA, found depriving animals and humans of sleep, or awakening them during REM did not impair their ability to form long-term memories.

Sleep patterns (REM in total hours)

Platypus - 8 REM in 14 Ferret - 6 REM in 14.5 European hedgehog - 3.5 REM in 10.1 Dolphin - 0.2 in 10 Humans - 2 in 8

Professor Siegal said: "Sleep loss interferes with concentration. For this reason, cramming for exams during periods of sleep loss is not a good academic strategy.

"Similarly, taking exams while sleepy will interfere with performance.

"However, it does not appear that sleep is required for memory formation."

He also found the time spent in REM sleep did not correlate with learning ability in humans or in other species.

Dolphins 'little REM'

The duck-billed platypus is one animal which has a high amount of REM sleep, but is not especially smart.

But dolphins and whales, which are known to be intelligent, have little or no REM sleep.

Dreaming is our mind watching our brain as it goes through and reviews, and tries to recombine memories from both the recent and the distant past

Dr Robert Stickgold, Harvard University

In comparison to other species, humans do not have a particularly high or particularly low amount of REM sleep.

Professor Siegal also looked at research on people taking antidepressants called monoamine-oxidase inhibitors, which eliminates REM sleep for months or years.

These people have unimpaired, or even improved memory.

He said people with brain damage that prevents sleep have normal memories.

Processing

Dr Stickgold said he believed the brain received too much information to process during waking hours.

"I think what dreaming is, is its our mind if you will, watching our brain as it goes through and reviews and tries to recombine memories from both the recent and the distant past."

But he said it was still a mystery why dreams told stories.

Dr Stickgold said the brain was active all through the night, but what he called 'dream reports' varied in tests between the various stages.

He added that the reason people did not remember dreaming when they awoke was probably due to the transition between sleeping and being awake.