Photo: Nicole Hill/Corbis

Anyone who has ever driven home after a red-eye flight or sat through an important but interminable lecture will know the feeling: You’re commanding your brain to stay awake, yet your eyelids feel heavy as lead and they just keep falling shut.

Now a team of neuroscientists based in Singapore has documented what’s going on in the brain when the battle to stay awake is momentarily lost. Ju Lynn Ong and her colleagues put 18 healthy participants in a sleepy state by keeping them awake all night in a psych lab. After being awake for 22 straight hours, the participants were asked to lay in the dark inside a brain scanner for two six-minute scans — and to keep their eyes open the whole time.

Needless to say, the participants’ eyes frequently drifted shut, and whenever that happened they were prompted by recorded messages to keep their eyes open. The researchers looked specifically at the patterns of activity in the participants’ brains in those moments that their eyes fell involuntarily shut. To make sure that any observed neural activity wasn’t simply a consequence of the act of closing the eyes, a comparison scanning session was performed in which the same participants were well-rested and simply instructed to close their eyes briefly at intermittent intervals.

When participants’ eyelids fell shut involuntarily and momentarily — some researchers call this a “micro-sleep” — one neural hallmark was a reduction in activation in the thalamus, a structure beneath the cortex that is often described as the brain’s relay station. It acts like a gateway, filtering the sensory information that’s sent to higher brain areas for further processing. This makes sense. As their wakefulness slipped away, the participants were shutting down their connection to the outside world.

More surprising – since we usually think of sleep as a restful activity – the participants’ involuntary eye closures were also associated with a dramatic uptick in activity across multiple regions, including areas involved in processing sensory information, moving the body, and the emotions, including self-awareness and memory (conversely, other areas, such as the anterior cingulate cortex — sometimes nicknamed the “oh shit circuit” for its role in error monitoring — showed a reduction in activity). More surprising still, the researchers found micro-sleeps triggered extra joint activation of frontal and more rearward brain areas involved in paying attention. This same combination of brain changes was not observed when participants were rested and chose to close their eyes at will.

What to make of all the neural shenanigans that occur when we lose the fight to stay awake? Previous research has described how, if you wake someone just after they’ve drifted off, they will often say they were experiencing strange, vivid feelings and sights — what’s often referred to as a hypnagogic state. This is likely the consequence of the dramatic spread of activity that was seen in this study across multiple sensory and emotional regions upon involuntary eye closure. As you slip out of consciousness, your brain puts on an instant dream show. However, it’s probably best to think of this as your brain’s B-movie equivalent to a full-on REM (rapid eye movement) dream. A study from the ‘90s repeatedly woke participants just as they dropped off to sleep, to ask them about their hypnagogic experiences: Only a minority included all the wacky wonders of normal dreams in terms of intricate plots and hallucinatory special effects.

Still, with such a rapid shift from the real world to our inner cinema, it’s little wonder we often feel so disorientated upon sudden re-awakening. Meanwhile, the researchers said the surprising spike in activity in regions associated with attentional control probably reflects the brain’s attempt to heed its owner’s futile commands to stay awake. This is a good example of how, at any given time, our conscious experience is the result of hidden, internal neural tugs-of-war — what we pay attention to, what we remember, how we choose, and in this case whether we actually stay awake, all comes down to the tussle between competing neural networks.

Unfortunately there aren’t any practical lessons here for how to avoid losing the sleepiness battle (save ensuring you aren’t too sleep-deprived), but it’s fascinating to have an insight into the way our brains explode with activity the very moment that “we” drop off to sleep.

Dr. Christian Jarrett is editor of the British Psychological Society’s Research Digest blog. His latest book is Great Myths of the Brain.