Dr. Anita Lüthi, Department of Basic Neuroscience, University of Lausanne, Switzerland: Falling asleep, i.e., the transition from wakefulness to sleep, is a dynamic process in which the brain plays an important role. The brain is divided into different areas that are responsible for different functions, such as enabling us to move, feel things, and be aware of them. These areas of the brain are progressively involved in the process of falling asleep. The brain areas do not all switch from zero to one hundred into the state of sleep, but rather adjust to it gradually. This is why the transition to sleep does not take place in all areas of the brain at the same time.

The fact that the process of falling asleep is not noticed indicates two things: on the one hand, there are areas of the brain that are responsible for our consciousness and that transition relatively quickly into the sleep state. On the other hand, there are indications that areas of the brain involved in memory formation ‘fall asleep’ relatively quickly. The perception of falling asleep is therefore a double-edged sword: although certain aspects of the process of falling asleep are perceived, the person falling asleep does not remember them afterwards. Dreaming offers a good comparison: surely everyone has had the experience of being aware of a dream from the previous night but not remembering its exact content. This is because during the dream, areas of the brain that process visual or auditory experiences or content became ‘awake’ – areas of the brain that are involved in memory formation, but not at the same time.

Surely everyone has fallen asleep during a lecture or in front of the TV at some point. Many would also agree that they were at least partially aware of the process of falling asleep. Or if you watch someone falling asleep while sitting, their eyelids close first and their head drops forward. And at some point, there is a twitch and the head straightens up again. Falling asleep does not only affect the brain, but also the muscles, which relax. This is referred to as decreasing muscle tone. When falling asleep while lying down, muscle twitches, for example in the legs, may also occur. This is caused by the muscles relaxing and the body then reflexively trying to regain tension. The decrease in muscle tone is very important in ensuring that we move as little as possible during sleep and, above all, that we do not walk around and injure ourselves. The horror stories of sleepwalkers illustrate this problem.

Physiologically speaking, the process of falling asleep is a gradual change in the electrical activity of the brain. Compared to the waking state, larger and slower electrical waves are generated and spread throughout the brain. As soon as these waves reach the areas of the brain that regulate our consciousness, our perception of our surroundings and ourselves diminishes. However, we are anything but unconscious when we sleep. A mother can still hear her child whimpering even when she is asleep. And we have probably all been woken up by unusual noises, for example from wooden walls in the bedroom.

The physiological understanding of sleep clearly shows that sleep should not be understood as a global brain state, but rather as an initially local and secondarily globalized brain state. There is evidence that certain areas of the brain can already sleep while others are still awake. When you are very tired but still awake, certain areas of the brain are no longer as active as others, meaning they are already transitioning into the sleep state. However, this process can happen very quickly – microsleep in overtired drivers can lead to reduced perception of the environment in a fraction of a second.

Another explanation can be found by looking at the areas of the brain that have a direct influence on the transition from wakefulness to sleep. In the brain stem, located at the back of our head, several areas of the brain are responsible for keeping us awake, alerting us, motivating us, making us exert ourselves, and making us sensitive. These are very small areas of the brain, but they project far into the forebrain, where they stimulate sensory and cognitive processes. When these areas of the brain reduce their activity after a long day, this is part of a normal regulatory process controlled by our internal clock. This also leads to changes in the activity of the thalamus, an egg-shaped area of the brain located beneath our cerebral cortex. The thalamus transmits sensory information, such as light and sound impulses, to the cortex. There is evidence that the activity of the thalamus decreases during the process of falling asleep, preventing sensory information from reaching the cortex. This deactivation is associated with a change in consciousness, i.e., the fact that one no longer perceives every sensory stimulus and falls asleep.

Recorded by Stefanie Flunkert