How a Human's Brain and Body Changes While Falling Asleep

We may be getting a new view of sleep. Scientists have developed a system to accurately track the dynamic process of falling asleep, something that has not been possible in the past with existing techniques.

"While our personal experience tells us that falling asleep is a gradual process, current clinical methods only define a single point in time at which one has fallen asleep," said Michael Prerau, lead author of the new study, in a news release. "Our new research shows that it's not simply when you fall asleep that's important, it's how you fall asleep that really matters. We now have the power to chart the entire trajectory of your neurological, physiological and behavioral activity as you transition from wake to asleep, rather than simply reporting the time it takes."

In order to link changes in brain activity to loss of consciousness during sleep onset, the researchers developed a new, minimally disruptive means of tracking behavior as someone falls asleep. Earlier methods used tasks in which a participant was asked to respond to auditory cues, something that could disrupt falling asleep, or actigraphy-measuring movement used in most clinical sleep devices which cannot distinguish between sleep and motionless wakefulness.

The new method involves a new behavioral task. Instead of responding to sound, participants hold a rubber "stress ball" in one hand and squeeze the ball with every intake of breath and release it when exhaling. A special glove on the squeezing hand and electrodes on the forearm measure the timing and the force of each squeeze. This allows the researchers to track sleep onset.

While the ball-squeeze measurements are taken, EEG readings track three brain wave patterns associated with falling asleep, decreasing power in the alpha frequency range and increasing power in the delta and theta frequencies. The combination of these measures is then used to calculate what the investigators call the wake probability, which is an estimate of the degree to which a participant is awake during the process of sleep onset.

"These participants continued to respond to the task, even though current clinical measures would say that they were still asleep, which was clearly not the case," said Prerau. "These results suggest that it is the presence of delta and theta power, rather than the lack of alpha power, that is necessary for the cessation of behavior. We may need to carefully re-examine the way sleep onset is defined, since behavior is an essential component of the story that is not measure clinically."

The findings are published in the journal PLOS Computational Biology.

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