Scientists have identified two "dreaming genes" that regulate how much we dream.
The discovery sheds light on the mystery of REM (Rapid Eye Movement) sleep - the phase of sleep during which most dreaming takes place.
Both humans and animals dream, but scientists are still trying to understand what, if any, function dreaming has.
One school of thought says dreams are simply a by-product of brain activity during sleep.
Another suggests they might have a necessary function, such as helping the brain archive important memories or rehearse challenging scenarios.
During REM sleep, which occurs at intervals during the night, the brain is as active as it is when awake.
Now a team of Japanese scientists has located two genes that appear to switch the REM dreaming state on.
In mice, REM sleep was reduced to almost undetectable levels when both genes were deactivated.
The genes code for two "receptor" proteins, Chrm1 and Chrm3, that produce a biological response when exposed to the neurotransmitter acetylcholine.
Neurotransmitters are chemicals that allow nerve signals to pass between neurons.
When both genes were "knocked out", the mice almost completely stopped experiencing REM sleep, but appeared unharmed by the experience.
Lead researcher Dr Hiroki Ueda, from the University of Tokyo, said: "The discovery that Chrm1 and Chrm3 play a key role in REM sleep opens the way to studying its underlying cellular and molecular mechanisms and will eventually allow us to define the state of REM sleep, which has been paradoxical and mysterious since its original report."
Writing in the journal Cell Reports, the scientists said the research would help to show whether REM sleep and dreaming plays a role in learning and memory.
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