How did you sleep through your alarm? TAU study reveals how deep we sleep
During deep sleep, unless a sound is powerful enough like an alarm clock, or instinctually meaningful like a baby's cry, our brain will rarely acknowledge them
By ZACHARY KEYSERLife in lockdown: Chiara Zuddas, 31, sleeps in bed cuddling her daughter, two-year-old Bianca Toniolo, at home in San Fiorano, one of the original 'red zone' towns in northern Italy that has been extended to the whole country(photo credit: MARZIO TONIOLO/VIA REUTERS)
Have you ever wondered how you can sleep through a train passing by your house at 3 a.m., or how some people can live on the busiest streets and still get a good night sleep? Or maybe even why those five alarms you set for the morning didn't capture your attention in the slightest.Well, researchers at Tel Aviv University (TAU) have figured out what controls how deeply we sleep.The study, led by TAU doctoral student Hanna Hayat of Prof. Yuval Nir's lab, intended to explore the reasons why, during sleep, we rarely respond to external stimuli such as sounds, even though our brain stays quite active.During deep sleep, unless a sound is powerful enough like an alarm clock, or instinctively meaningful like a baby's cry, our brain will rarely acknowledge it - allowing us to stay at rest, which is conducive to a healthy lifestyle.However, there is some contrast to a one-size-fits-all sleep - there are light sleepers, who can be awoken by the faintest sounds like a creak in the floorboard, and there are deep sleepers, who could more or less have a truck drive into their house and still carry on sleeping.According to their findings, published in thejournal Science Advances, the researchers found that an area of the brain stem called the locus coeruleus - responsible for secreting the chemical noradrenaline during times of stress - is the main component in blocking sensory responses while at rest, allowing us to sleep soundly throughout the night."The ability to disconnect from the environment, in a reversible way, is a central feature of sleep,” explains Hayat. “Our findings clearly show that the locus coeruleus noradrenaline system plays a crucial role in this disconnection by keeping a very low level of activity during sleep."It is noted that given the fact the locus coeruleus and the secreted chemical noradrenaline are connected to stress, a person's mental state can be the key to how responsive our sensory system will be to external stimuli while at rest - and in turn how good of a night sleep that person will be able to obtain.To arrive at their conclusion, the researchers used rat models to determine the activity levels of the locus coeruleus during sleep while introducing them to an array of sounds to see which, if any, would awaken the subjects. Within that, they found that the activity level of the locus coeruleus directly correlated to the animal's response to the external stimuli, and could accurately predict if the rodent would awaken by being introduced to certain sounds.Through optogenetics, the researchers tested this theory, by silencing the locus coeruleus using light to control the neuronal activity - to which they found the rats would not awaken to the introduced sounds.
"When we increased the noradrenaline activity of the locus coeruleus while a sound played in the background, the rats woke up more frequently in response, but when we decreased the activity of the locus coeruleus and played the same sound in the background, the rats rarely woke up,” said Hayat. “So, we can say that we identified a powerful ‘dial’ that controls the depth of sleep despite external stimuli.”“Sleep disturbances are a major health issue and are frequent in aging, as well as in neurological and psychiatric disorders,” Nir concluded. “It is important to test if our findings on varying noradrenaline levels can explain hyperarousal that characterizes condition such as anxiety disorders and PTSD, and if so, to build on these findings to develop novel methods to improve sleep quality.”
