PARIS — Some people replay the events of their day, sometimes imagining how things could have gone differently. Others plan for tomorrow, letting themselves drift off to a mix of images, sounds and fragments of dreams. What do you think about as you fall asleep?
That question lies at the center of a three-year research project at the Brain Institute of the Pitié-Salpêtrière Hospital in Paris. Unlike REM sleep — a stage known for vivid dreaming and long studied by scientists — the moment of drifting into sleep has received far less attention.
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“This brief phase, known as N1, is a transitional state between wakefulness and light sleep,” explains Nicolas Decat, a doctoral student in cognitive neuroscience who launched the project. “It’s rich in content, but hard to study because people rarely remember it.”
To capture the diversity of experiences during these few minutes, the researcher plans to interview 5,000 people from different cultural backgrounds. He has created a 20-minute questionnaire, available in French, English and Spanish, that explores the wide range of feelings people report as they drift off to sleep. So far, he has received 600 responses. His aim is to uncover recurring patterns of thought in this gray zone between wakefulness and sleep, and to map out different sleeper profiles.
“I had music in my head”
It was during earlier research that Decat first became aware of the “richness of mental content” in the moments before sleep. The first stage of his doctoral work, carried out last year, involved about 100 volunteers in two rooms dedicated to research at the sleep disorders department of the Pitié-Salpêtrière Hospital.
Seated in reclining chairs and fitted with headsets to record brain activity, participants were asked to take two 20-minute naps. They were woken up several times and asked to describe their thoughts “in the heat of the moment.” To do this, scientists used what is known as the “bottle test”: volunteers held an object in their hand, which would drop and wake them just as they began to drift off.
The material Decat collected was strikingly varied: “I had music in my head.” “I was thinking about my trip to Japan last year.” “I saw images of little aliens.”
During stage N1, the heart rate slows, the muscles relax, and awareness of the outside world begins to fade. In the brain, electrical activity shifts in intensity from one region to another. It is in this state that dreamlike impressions or distorted thoughts arise — what scientists call hypnagogic perceptions. These can be shaped by what a person was doing before sleep — for instance, seeing bricks tumble from the sky after playing hours of Tetris — or by external stimuli, such as sounds drifting in from the street.
“Using machine learning, we were able to classify mental content into four broad categories: memories, attention to the environment, dreams, and pragmatic thoughts,” Decat explains. These different forms of thinking can appear in the same nap, in varying order — for instance, everyday reflections, followed by a loss of control and the sudden arrival of strange images. By analyzing electroencephalogram (EEG) recordings, the researcher also discovered that each of these thought types has its own distinctive brain signature.
Sleep patterns
Through this research, the Brain Institute team — nicknamed the “Dream Team,” composed of three lead investigators and about 20 young researchers — hopes to shed light on the mechanisms of sleep disorders, particularly insomnia and excessive daytime sleepiness, both of which involve disruptions at the moment of falling asleep.
“In general, it would be useful to know whether patterns at sleep onset are linked to the quality of sleep and recovery,” Decat says. “Do people who describe themselves as insomniacs experience the same kinds of mental content? And what about those who are highly creative or prone to anxiety?” If so, analyzing the flow of thoughts at the threshold of sleep might one day make it possible to predict personality traits — with important clinical applications.
The period of falling asleep plays an important role in attention, memory and emotion regulation,”
We know that the N1 stage can spark creativity, often surfacing upon waking. In another study, a large majority of participants reported having useful ideas or solutions to daytime problems while in a state of “half-sleep.” Research also shows that resting for a few minutes in the dark, which encourages the onset of sleep, can enhance problem-solving abilities.
“Overall, the period of falling asleep is not only a gateway to the many benefits of the deeper sleep that follows, but it also plays an important role in various cognitive processes, including attention, memory and emotion regulation,” say the authors of a literature review published last year.
The topic is more important than it might seem. Researchers note that the transition from wakefulness to sleep does not only happen at the start of the night, but also when waking after a long sleep and during daytime rest periods. This state, therefore, occurs more often in our 24-hour cycle than we might realize.
