When we sleep there are three or four different stages of sleep that we pass through on any given night. These are broadly separated into two different types - non-rapid eye movement (Non-REM) and rapid eye movement (REM).
When we fall asleep we go through stages one, two and three of Non-REM sleep. This involves gradually getting into deeper and deeper sleep, so you become more and more unconscious and less aware of your environment. Typically there’s not much dreaming going on during these Non-REM periods.
"The dreams you remember when you wake up typically occur during REM sleep."
After these three stages and into deeper sleep, we progress back up into lighter sleep. When you get back to stage one you’ll hit a period of REM sleep and this is the period of sleep that is most important when it comes to understanding dreams. This is where most of the vivid dreaming occurs - the dreams you remember when you wake up typically occur during REM sleep. That’s the stage of sleep that’s most linked to dreaming.
When you’re in rapid eye movement sleep the metabolic activity of the brain is very high - it’s very similar to when you’re awake - especially when compared to Non-REM sleep when your brain is typically less active.
During REM sleep your muscles experience a form of paralysis. This helps to prevent you acting out your dreams. There’s actually a disorder, called REM behaviour disorder, where the paralysis doesn’t occur and people will act out their dreams. They can move their arms about, kick out and flail about. It’s pretty uncommon and is actually a pretty good predictor of neurodegenerative disorders such as Alzheimer’s - there’s a higher prevalence of people that suffer from this behaviour disorder who will go onto develop these conditions.
We think this REM activity is generated in the brain stem - which is situated right at the bottom of your brain. It’s quite an ancient structure, evolutionary speaking. It can be found across lots of animals. We actually think REM sleep occurs across all mammals and birds as well. So that activity is found there.
"There are areas of our brain that are active when we’re daydreaming and also when we’re dreaming, and some people have suggested that there’s quite a lot of similarity between the content of a dream and a daydream."
It’s called rapid eye movement because your eyes move about really fast - and this occurs when you’re awake too; they’re called saccadic eye movements, they help with perception. And the systems that underlie this when you’re awake also appear to control REM when you’re asleep - so the bits that control the muscular movements of the eyes are active when you’re asleep. The visual cortex is probably where the dreams are generated.
There are areas of our brain that are active when we’re daydreaming and also when we’re dreaming, and some people have suggested that there’s quite a lot of similarity between the content of a dream and a daydream. The major difference is a lack of activity in the frontal areas of our brain, particularly in the prefrontal cortex during a dream - and this is an area of our brain that is involved in the higher order cognitive systems, like our sense of self. That’s geared towards sleep, which probably explains why the narrative in a dream is quite incoherent.