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How are spontaneous thoughts and sleep related?

By Nicolas Scrutton Alvarado

A woman face down on a desk, lying on top of a pile of books. There is a a vase with red flowers in the background.
Struggling to concentrate? It may be due to a bad night's rest

We all know what it is like to not be able to focus. It may be on a long drive, it may be in a boring lecture, it may be during a morning meditation. Random thoughts arise spontaneously – did I switch off the lights? I shouldn’t have had that last piece of cake. I wonder how Jim is doing.

At best, these thoughts inspire your next great idea. At worst, they can be a barrage of distractions that you just can’t get out of your head. Scientists are still working out exactly why these spontaneous thoughts occur, but some research suggests a bad night’s rest may just have something to do with it.

First, let’s figure out what a spontaneous actually is. The definition itself is something of a debate in the neuroscience community. For some, spontaneous thoughts are defined by what they are not – directed thought, like the one needed to read this article.

Others argue the definition is too vague, too broad. They define spontaneous thought using a framework of automatic vs deliberate constraints. Automatic constraints on thought could be worries outside our immediate attention, like worry over a coming deadline. Deliberate constraints, on the other hand, are consciously imposed, like the attention required to listen to a lecture.

Thoughts with low deliberate constraints but high automatic constraints are thought of as ruminations, or obsessive thoughts. Thoughts with deliberate constraints but low automatic constraints are thought of as goal-directed, or focused thought. Using this framework, spontaneous thoughts are those with low automatic and low deliberate constraints – things like creative thinking, dreaming, and mind wandering.

How spontaneous thoughts are defined, modeled after Christoff et al, 2016; Nature Reviews Neuroscience

These spontaneous thoughts can be further placed into positive or negative categories. “We have positive-constructive daydreaming, which relates to pleasant thoughts about the future, or problem solving” said Dr. Ana Lucía Cárdenas-Egúsquiza, a postdoctoral scholar in the department of psychology and behavioral sciences at Aarhus University in Denmark. Those are the great ideas that come in the middle of the night, or happy thoughts about your plans for the weekend.

There’s also a category called disruptive mind wandering and daydreaming. “They are related to more negative emotions,” said Dr. Cárdenas-Egúsquiza. Those are the thoughts that distract you from whatever you’re trying to focus on, thoughts centered on fears and guilt.

So spontaneous thoughts are those which we have little deliberate or automatic control over, and they fall into either positive or negative categories. What does sleep have to do with it?

Research suggests that a bad night’s sleep is correlated to an increase in disruptive daydreaming and mind wandering – the negative forms of spontaneous thought. Bad nights stemming from poor sleep quality, sleep disturbances, and sleep deprivation were all related to disruptive spontaneous thoughts. Sleep duration, on the other hand, did not seem to affect the amount of spontaneous thoughts.

Interestingly, not all kinds of spontaneous thoughts were affected the same way. Positive-constructive spontaneous thoughts – the great ideas, the happy daydreams – did not seem to be correlated with a bad nights’ rest.

How sleep quality and spontaneous thoughts are linked is still unclear. There are a few hypotheses researchers are actively studying.

For example, spontaneous thoughts may come from our stream of consciousness, the continuous thread of thoughts that accompany our everyday lives. When we focus on a task, psychologists theorize we use executive control to inhibit this stream of thoughts.

Executive control – the mental process we use to concentrate, plan, and think things through – requires energy to use. Studies in laboratory settings have observed that reduced executive control was associated with increases in daydreaming and mind wandering.

A bad night’s rest, somewhat unsurprisingly, has been associated with reduced executive control. We may feel tired the next morning, low on energy, possibly impacting our ability to exert executive control over our thoughts. No matter how hard you try to focus, spontaneous thoughts continue to appear.

Our mental health, too, may play a role. Depression and anxiety have both been associated with reduced sleep quality and an increase in daydreaming and mind wandering. Insomnia and certain types of spontaneous thought, particularly disruptive mind-wandering and thoughts of failure, can be considered a defining characteristic of many of these disorders.

It’s important to note, however, that none of these studies state that a bad night’s rest is causing an increase in spontaneous thoughts. “The other way could be true as well,” said Dr. Cárdenas-Egúsquiza. A day full of disruptive mind wandering, filled with thoughts of failure and guilt, could also affect your sleep that night. “I think it’s bidirectional,” she said.

Our thoughts accompany us wherever we go, whatever we do. They can appear at unexpected times, potentially offering a source of brilliance, potentially distracting us from a critical task. How and why exactly these spontaneous thoughts occur remains something of an ongoing mystery. It may just be that last night’s rest may hold the answer.


Thank you to our community member who asked this question! If you have any questions about mental health, send them over to our questions tab or reach out to us on Instagram or Twitter!


If you’re interested, check out some of the sources I used in researching this answer! All sources used for this answer are strictly evidence-based.

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