Entering deep sleep and then waking up before completing the normal sleep cycle can result in a phenomenon known as sleep inertia. New brain scan research in NeuroImage helps explain why this phenomenon is associated with reduced cognitive performance.

“My former PhD advisor (Dr. Perrine Ruby) and I were initially interested in sleep inertia — and what happens in the brain during this transient period — because we wanted to understand how this could relate to the recall or forgetting of dreams, which was at the time the main topic of my PhD,” said study author Raphael Vallat of the Center for Human Sleep Science at the University of California, Berkeley.

“And, while we were planning the study, we realized that there was in fact no study that had previously looked at sleep inertia using a multi-modal approach (i.e. using both electrophysiological (EEG), behavioral and neuroimaging (fMRI) approaches).”

“In other words, while the behavioral aspects of sleep inertia were well-described in the scientific literature, very little work had been done on the neurophysiological correlates of sleep inertia,” Vallat said.

The researchers measured the cognitive functioning and brain activity of 34 participants before and after a 45-minute nap.

They found that cognitive performance, as measured with a mental subtraction task, was reduced shortly after awakening from the nap. Participants who were in a deeper sleep before awakening tended to have even bigger drops in performance. But the negative effects dissipated about 25 minutes after awakening.

This decrease in cognitive performance during sleep inertia was correlated with increased delta brainwave activity. In addition, Vallat and his colleagues found that the functional connectivity between brain networks was strongly disrupted shortly after awakening from the nap.

“Awakening in deep sleep is associated with strong alterations in brain function and mental performances that last for at least 10 to 20 minutes,” Vallat told PsyPost.

“Since we know that the average latency to reach deep sleep is about 25 to 30 minutes, a public health recommendation would be that people should either take short naps (less than 25 minutes) to avoid diving into deep sleep or full, 90-minute naps to do a full sleep cycle.”

“I think that people should also be more informed about sleep inertia and try to be aware of its negative effect whenever they have to make important decisions or drive in the first minutes after awakening,” Vallat explained.

But researchers still have much to learn about sleep inertia. Future research could address a number of unanswered questions.

“The study of the neurophysiological aspects of sleep inertia is still at its infancy, and many questions remain unsolved. For instance, we only studied sleep inertia after awakening from N2 sleep and deep (N3) sleep in our study, and it would be interesting to see if our results replicate for REM sleep and N1 sleep,” Vallat said.

“That said, we know from behavioral studies that sleep inertia is generally weaker after awakening from these two sleep stages, which are functionally closer to wakefulness.”

“More research should be done to find ways to counteract the negative effect of sleep inertia. For instance, we know that the ingestion of caffeine before napping can limit the downside of sleep inertia but we do not really know what are the mechanisms for that,” Vallat added.

The study, “Hard to wake up? The cerebral correlates of sleep inertia assessed using combined behavioral, EEG and fMRI measures“, Raphael Vallat, David Meunier, Alain, Nicolas, and Perrine Ruby.