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We tend to think of falling asleep or waking as an either-or-perspective: when you're not sleeping, you need to be awake. But the sleep disorder specialist and neurologist Guy Leschziner says it's not so easy.
"When you look at the brain as you sleep, we now know that sleep is not a static state," says Leschziner. "There are a number of different brain states that occur while we sleep."
As head of the Sleep Disorders Center at Guy's Hospital in London, Leschziner has treated patients with a variety of nocturnal problems, including insomnia, night anxiety and narcolepsy, sleepwalking, sleeping and sex, a condition in which a person is in bed Sleep performs sexual acts. He writes about his experiences in his book The Nocturnal Brain.
Leschziner notes that the different parts of the brain are not always in the same sleep at the same time. In this case, a person can order a pizza or drive a car – while technically still sleeping deeply.
"Sometimes these conditions sound very funny," says Leschziner. "But in other cases, they can really be life-changing and lead to serious injury or, as in any of the cases described in this book, to a criminal conviction."
Highlights of the Interview
From What We Know About Memories of a Sleepwalking Episode
We used to think that people do not remember anything that happens at this stage , This seems to have something to do with the fact that the brain partially sleeps very deeply while it is awake in other parts. What we have learned in recent years is that, in fact, quite a few people have some kind of limited recall. They do not necessarily remember the details of all events or the entirety of the event, but sometimes they experience little snippets. … One day [a patient] got his girlfriend out of bed in the middle of the night because he thought a tsunami would wash her away, and those events with a strong emotional context should often be remembered better.  As shown by sleepwalking, the brain can be in several sleep stages at the same time.
Certain parts of the brain can remain in very deep sleep. [such as] The frontal lobes, which are the seats of our rational thinking or planning or limiting to normal behavior, while other parts of the brain can show electrical activity that is really similar, as if we were wide awake. In particular, the parts of the brain responsible for the emotion [can seem to remain awake] [the ones] an area of the brain called the limbic system, obviously the parts of the brain responsible for the movement . And it is this dissociation, this separation between the various parts of the brain in relation to the sleep phases, that actually leads to such behaviors.
What causes sleepwalking
We know that sleepwalking and these related conditions are very common in families. So there seems to be some sort of genetic predisposition to be able to enter this separate brain state, and we know that anything that disturbs your sleep, if you have that genetic predisposition, can lead to these behaviors. For example, I've seen people who had non-REM parasomniac events [such as sleepwalking] who were triggered by sleeping in a creaky bed and their bed partner [or] sometimes rolling over a big truck [drove] outside the street bedroom.
But there are also inner manifestations, inner processes that can lead to this partial awakening. For example, snoring or, more than snoring, sleep apnea, where people stop breathing when they are asleep … all that can lead to a change in sleep depth in people who are predisposed to this phenomenon of staying in multiple sleep stages these behaviors occur.
Sleep apnea describes the phenomenon that our airways are falling asleep. … Our airway is essentially a flaccid tube that has some rigidity and structure due to several muscles. And when we fall asleep, these muscles lose some tension and the airway gets a bit flaccid. If it is a bit slack now and it reverberates when inhaled, it snores – the reverberation of the airway walls leads to noise.
However, in some individuals, the airway may become slack enough or narrow enough to collapse and block the flow of air while we sleep. It is normal for this to happen to anyone from time to time, but if it happens very often, sleep can sometimes be interrupted 10, sometimes 20, sometimes even 100 times an hour, because we fall asleep, the respiratory tract collapses, and our oxygen levels drop Our heart rate is rising, our brain is awakening and our sleep is essentially disrupted. …
We now know that obstructive sleep apnea has a number of long-term effects on our health, such as: Hypertension, cardiovascular risk, stroke risk, effects on cognition and mental clarity. And there is now a body of evidence that suggests that obstructive sleep apnea may actually be a factor in the development of conditions such as dementia.
About the importance of having positive associations with your bed
When you sleep well, you tend to be in bed to be in that place of comfort where you go and feel … comfortable and you fall asleep and wake up wide awake and refreshed in the morning. But for people with insomnia, they often associate bed with great difficulty falling asleep, with the fear of the night, with the fact that they know that they will feel terribly refreshed and restless upon waking up. And so the environment we normally associate with sleep becomes a torture device to them. Many of the advances that have been made in this area with regard to the treatment of insomnia are aimed at destroying the negative associations of people with their sleep environment when they have insomnia and restoring positive associations. So, try to use the brain's own mechanisms to fall asleep and reduce sleep anxiety to restore a normal sleep pattern.
The problem of ingestion of benzodiazepines and  Ambien against insomnia
In recent years, there has been some change in these medicines. We know that these medications [are] are tranquilizers. The first thing you need to know is that they do not imitate normal sleep. They are associated with some big problems. For example, some of these drugs are associated with an increased risk of morning traffic accidents due to hangover. For example, they are associated with an increased risk of falling in the elderly. And we know that people can develop and get used to these drugs, which means they need more and more doses to get the same effect.
Long term, there are now some signals from worldwide work that suggest that some of these drugs are actually associated with an increased risk of cognitive decline and dementia. And although this story is not fully understood – and it may be that people who have insomnia in themselves are predisposed to dementia or that insomnia is indeed a very early warning sign of dementia – [it] certainly gives us cause for concern, maybe we too should not use these drugs as generously as we have done in the past. Moving to behavioral approaches, such as cognitive-behavioral therapy for insomnia, was therefore driven by some of these concerns not to read on a tablet or laptop, [and instead an] old-fashioned analog book, I would highly recommend. This is a great way to reduce the exposure. Keep your mind a bit active so you do not focus on the prospect of falling asleep until you're really tired. It's a great way to engage yourself.
Sam Briger and Mooj Zadie produced and edited the audio of this interview. Bridget Bentz and Molly Seavy-Nesper have adapted it for the web.