You may well have spent hours wondering what your laptop is up to as it takes its time to boot up. Scientists have asked the same question of the human brain: How exactly does it restart after being anesthetized, in a coma, or in a deep sleep?

Using a group of 30 healthy adults who were anesthetized for three hours, and a group of 30 healthy adults who weren't as a control measure, a 2021 study reveals some insights into how the brain drags itself back into consciousness.

It turns out that the brain switches back on one section at a time, rather than all at once – and abstract problem-solving capabilities, as handled by the prefrontal cortex, are the functions that come back online the quickest. Other brain areas, including those managing reaction time and attention, take longer.

"Although initially surprising, it makes sense in evolutionary terms that higher cognition needs to recover early," said anesthesiologist Max Kelz, from the University of Pennsylvania.

"If, for example, someone was waking up to a threat, structures like the prefrontal cortex would be important for categorizing the situation and generating an action plan."

A variety of methods were used to measure what was happening in the brain, including electroencephalography (EEG) scans and cognitive tests before and after going under. These tests measured reaction speed, memory recall, and other skills.

Analyzing the EEG readings, the researchers noted that the frontal regions of the brain – where functions including problem-solving, memory, and motor control are located – became particularly active as the brain began to recover.

A comparison with the control group showed that it took about three hours for those who had been anesthetized to recover fully.

The team also followed up with the group participants about their sleep schedules in the days after the experiment. The experience didn't appear to negatively affect sleeping patterns in those who had been anesthetized.

"This suggests that the healthy human brain is resilient, even with a prolonged exposure to deep anesthesia," said anesthesiologist Michael Avidan, from Washington University.

"Clinically, this implies that some of the disorders of cognition that we often see for days or even weeks during recovery from anesthesia and surgery – such as delirium – might be attributable to factors other than lingering effects of anesthetic drugs on the brain."

A lot of surgical procedures simply wouldn't be possible without anesthesia, an effective and controlled way of turning off consciousness in the brain – something that can happen involuntarily in the case of a coma.

Despite their widespread use, we don't really understand how anesthetics work in precise detail, even if we have figured out how to use them safely. There are plenty of ideas about how the brain deals with these drugs, but no concrete evidence as yet.

The recent findings can not only help with treatments and patient care – after major operations involving anesthesia, for example – but also in giving scientists a better understanding of the brain and how it responds to disruption.

"How the brain recovers from states of unconsciousness is important clinically but also gives us insight into the neural basis of consciousness itself," said anesthesiologist George Mashour, from the University of Michigan.

The research was published in eLife.