Each night when we drift off to sleep, our consciousness recedes, and the often bizarre parade of dreams begins. We know that this nightly medley of images and stories helps us process emotions and events. And yet, there is still much mystery to the world of dreams, including the mechanisms involved, the amount of time we dream, and even the full purpose of dreams.
Like dreams themselves, the answers, based on the research to date, are rather surprising.
How much time we spend dreaming
Dreams are commonly thought of as a hallmark of rapid eye movement (REM) sleep, the stage in which we spend about a quarter of our total sleep time. During REM sleep, which occurs following the various non-REM stages of sleep, our eyes are rapidly moving back and forth under our eyelids (hence the name). But our bodies are temporarily immobilized — a state known as muscle atonia. This is what keeps our bodies from acting out our dreams.
According to reports, when people are awakened from REM sleep, about 70 to 80% of our REM time is spent dreaming in a form that we’re able to recall, says Robert Stickgold, Ph.D., professor of psychiatry at Harvard Medical School, director of the Center for Sleep and Cognition at Beth Israel Deaconess Medical Center in Boston and co-author of “When Brains Dream.”
However, we actually dream far more than that. “We’re also dreaming more than half the time in non-REM sleep,” Stickgold explains, “My guess would be about 70% of the night, we’re dreaming.”
We dream in all stages of sleep
Most research on dreams involves rousing people from sleep to record their feedback. We know, based on these self-reports, that dreams aren’t the sole domain of REM.
“There is no sleep stage in which we don't get dream reports if we wake people up enough,” Stickgold says. In fact, as he and co-author Antonio Zadra write in “When Brains Dream,” the majority of the time, people awakened from any sleep stage (the three non-REM stages or REM itself) will report that they’d been dreaming.
The misconception that dreams only happen during REM dates back to the discovery of REM sleep based on the criteria adopted at that point about what constituted a dream. “They set this very high bar,” he explained. “It had to be narrative; it had to be emotional; it had to be highly visual; there had to be action going on. It had to be a really complex, full-blown dream to get counted.”
If using this standard, dreams tend to come only in REM sleep, Stickgold says. “But if you take those lesser dreams, then we’re dreaming certainly most of the night.”
Even though the first non-REM stage of sleep, known as transitional sleep, lasts only a few minutes, that’s still enough time for us to dream. Our dreams during this stage are often based on what we were thinking about just before we drifted off to sleep. These dreams are generally not as complex as later-stage dreams and may not contain a narrative story, Stickgold explains. “It’s not uncommon for the dream to really just be an image.”
As we pass from this transitional phase into Stage 2 sleep (known as light sleep), our dreams are also based on recent events. However, they’re not as long and strange as our REM-stage dreams, and it’s often easier to identify what sparked a particular dream, he says. Other than that, “Stage 2 dreams look very much like REM dreams.”
The deep, slow-wave sleep of Stage 3 makes it more difficult to obtain dream reports, as people are frequently groggy when woken from a deep sleep. “But when we do,” says Stickgold, “they look not that different from Stage 2 dreams.”
As sleep progresses, your dreams get a little wilder
It’s during REM sleep that our dreams become even more fantastical and obscure. The brain is searching through associative memories rather than recent memories, he explains.
As the night progresses, though, our dreams do too. Our REM sleep is backloaded so that more of it takes place later in the night, and as this happens, the content of these dreams continues to become more fantastical.
But that’s not all: Throughout the night, our Stage 2 dreams become more like REM dreams, says Stickgold. “Your REM dreams become more vivid and complex and more emotional as the night goes on, and your non-REM dreams sort of chase after [them]. By the end of the night, your non-REM dreams are indistinguishable from REM dreams from earlier in the night.”
Dreams as the flip side of consciousness
In “When Brains Dream,” Stickgold and Zadra describe dreaming as “network exploration to understand possibilities,” or NEXTUP. “We think that basically the reason that we dream is that the brain is searching through networks of these weak associations,” Stickgold says, “to try to find associations to current concerns, things that happen during the day that you're still thinking about, associations that are potentially useful.”
This can only happen when our brains are offline and can sift through and evaluate all of the information and experiences of the day. This happens the whole time we’re asleep, not just when we’re dreaming, he pointed out, but our dreams allow us to find associations that we wouldn’t make otherwise.
When we’re awake, our prefrontal cortex, which is the seat of executive functioning, is very much in control. This allows rationality and reason to counterbalance or override impulsivity and emotional reactions. When we’re in REM sleep, though, the part of the prefrontal cortex that governs this “is just about completely shut down,” Stickgold says.
Meanwhile, activity increases in the limbic system and other parts of the brain involved in emotional processing, which is why our dreams tend to be far more emotional than logical.
There are also brain chemistry changes during REM, Stickgold explains, including a temporary block of the release of both serotonin and noradrenaline and an increase of acetylcholine. “All of those together cause the brain to preferentially activate weak associations,” he says.
It’s these weaker associations that provide the increasingly strange content of REM dreams, with the connections to real life being far less clear-cut than in non-REM dreams.
It may be, too, that our REM-sleep dreams allow us to separate the emotions associated with highly charged events from the content of those memories, says Patrick McNamara, Ph.D., associate professor of neurology at Boston University’s School of Medicine and author of “The Neuroscience of Sleep and Dreams.” This allows our brains to “dampen down” the arousal associated with the event, he said, while storing the content in our long-term memory.
We may also be processing emotions associated with these events via an “intense generation of counterfactuals,” he said. In this scenario, our brains use the original “highly charged emotional image” as the jumping-off point, then create more and more extreme versions with less and less apparent relation to the original, lessening its impact.
“The imaginative spinning of scenarios or mental simulations to some emotional event allows us to see ways in which the event could have gone differently, had initial conditions been ever so slightly different,” McNamara said. “And that info is invaluable for learning.”
However, whether we can interpret our dreams or even remember them isn’t essential.
Considering how much time we spend dreaming while asleep, the proportion of our dreams we’re able to recall is extremely low, which means “we have to have a function of dreaming that’s served while the dream is actually occurring,” says Stickgold. “That’s what we think is going on with this assessment of dream content by the brain looking for emotional reactions in these associative narratives and then strengthening synaptic pathways right then and there.”
Even for those dreams we manage to recall, understanding the meaning is far from straightforward. “It could be that there’s a subtlety involved,” Stickgold says, “that when we remember the dream upon waking up, we can’t see the part that’s important.”