When it comes to dreamlessness, conventional wisdom states that consciousness disappears when we fall into a deep, dreamless sleep. 

But researchers have come up with a new way to define the different ways that we experience dreamlessness, and say there's no evidence to suggest that our consciousness 'switches off' when we stop dreaming. In fact, they say the state of dreamlessness is way more complicated than we'd even imagined.

"[T]he idea that dreamless sleep is an unconscious state is not well-supported by the evidence," one of the researchers, Evan Thompson from the University of British Columbia in Canada, told Live Science.

Instead, he says the evidence points to the possibility of people having conscious experiences during all states of sleep - including deep sleep - and that could have implications for those accused of committing a crime while sleepwalking.

But first off, what exactly is dreamlessness?

Traditionally, dreamlessness is defined at that part of sleep that occurs between bouts of dreams - a time of deep sleep when your conscious experience is temporarily switched off. This is different from those times when you simply cannot remember your dreams once you've woken up.

As dream researchers from the University of California, Santa Cruz explain, most people over the age of 10 dream at least four to six times per night during a stage of sleep called REM, or Rapid Eye Movement. (Studies suggest that children under age 10 only dream during roughly 20 percent of their REM periods.)

Considering REM periods can vary in length from 5 to 10 minutes for the first REM period of the night to as long as 30-34 minutes later in the night, researchers have suggested that each dream is probably no longer than 34 minutes each. 

While there's some evidence that we can dream during the non-REM sleep that occurs 1 or 2 hours before waking up, if you're getting your 7 hours of sleep each night, that still leaves a lot of room for dreamlessness.

Thompson and his colleagues suggest that the traditional view of dreamless as being an unconscious state of deep sleep is far too simplistic, arguing that it's not a uniform state of unconsciousness, but actually includes a range of experiences involving certain stimuli and cognitive activity. 

They say dreamless can be experienced in one of three states: 

  • Non-Immersive Imagery and Sleep Thinking - When you're confronted with isolated or non-moving visual or auditory cues that lack the kind of "hallucinatory context, movement sensations, and propositional thought" experienced during a dream. In other words, thinking about or experiencing things without feeling immersed in them like you would in a dream state.
  • Perceptual Experiences and Bodily Sensations - This involves experiencing perceptions and bodily sensations - including those from the real world - but not in connection to any dream-like context. An example is hearing an alarm while you're asleep.
  • 'Selfless' States and Contentless Sleep Experiences - "[T[his state not only involves dreamless sleep, but also a certain amount of conscious awareness on the part of the person that he or she is sleeping," Stephanie Bucklin from Live Science explains.

By categorising dreamless sleep like this for the first time, the researchers state that these experiences do not indicate a sudden 'switching off' of consciousness - just a shift in experience.

In fact, it's not yet clear if consciousness exists at all during sleep - just that it doesn't appear to be flicking on and off. As the researchers explain:

"This is not to claim that conscious mental activity continues throughout sleep. Whether periods of unconsciousness occur in sleep alongside dreaming and dreamless sleep experience is, in our view, an open empirical question.

The more pressing point is that, given the available evidence, it seems unlikely that all stages of sleep currently classed as dreamless necessarily or consistently involve a loss of phenomenal consciousness."

This also suggests that we may need to rethink how we define crimes committed during sleepwalking - such as homicidal sleepwalking. 

It's been argued that culpability is lacking if the person has no experience, perception, or mental interaction with the environment - or memory of the crime afterwards - but the researchers suggest that, based on the experiences you can actually have during dreamlessness, this kind of behaviour might not necessarily be unconscious. 

"In our view … requiring non-REM parasomnias [or sleep disorders] to be completely unconscious … seems too strong," they conclude.

Dreams and dreamlessness are incredibly difficult things for researchers to study and define. We still don't know why we dream, if dreams have meaning, and why some of us struggle to remember any of them.

But figuring out how much, or little, control our brains have during the various stages of sleep could help researchers treat all kinds of sleep disorders and behaviours, from sleepwalking - whether murderous or not - insomnia, and restless legs syndrome. 

"The exact relationship between sleep behaviour and sleep experience remains far more of an open question than is commonly assumed," says Thompson.

The research has been published in Trends in Cognitive Sciences.