Sleep Sense

Improve your sleep, improve your health

Dr Katharina Lederle

• Publisher: PublishDrive
• Year: 2021
• ISBN:
  9781775594802
• Topic: Medicina
• Subtopic: Medicina del sueño

Sleep is fundamental to our health and wellbeing. In Sleep Sense, international sleep expert Dr Katharina Lederle draws on the latest research to explore how sleep is connected to each of the three pillars of successful living: physical health, cognitive performance, and emotional wellbeing. She then provides readers with a 'menu' of healthy sleep options from which they can choose in order to effectively establish better sleep habits that improve day-to-day health and wellbeing.

PART 1

A SHORT TOUR

OF SLEEP

Sleep is the most exquisite invention.

— HEINRICH HEINE, 1856

1.

SLEEP: THE MOST FREQUENT QUESTIONS ANSWERED

When people hear I’m a sleep specialist, they ask me all sorts of questions about sleep: what is its role and function, what regulates it and how much sleep do we need? The best way to address these questions is to give you an understanding of what normal sleep is.

I’m going to take you on a sleep tour. Along the way we’ll stop off at some of the most interesting questions people ask, to give you a good overview of the topic. I’ll talk about what sleep is and how it comes about. I’ll explain what happens in your brain when you sleep. And why, for example, you feel sleepy after lunch. I hope to surprise you with some interesting facts and amuse you with some theories on what might help you sleep better. But, above all, I hope that reading this chapter will interest you enough to want to learn more about sleep, because I see this short sleep tour as the introduction to sleep on which all the other chapters of the book build. To get the most from this book, read this chapter first and stay on the guided tour — that way you won’t miss anything.

WHAT IS SLEEP AND WHY DO WE DO IT?

Sleep is a naturally occurring state that alternates with being awake. Typically, we spend one-third of the 24-hour day asleep and two-thirds awake, fully conscious of what’s going on around us. Sleep is a shift in consciousness. It’s a time when we ‘switch off’ from the world around us and become less responsive to it. However, that doesn’t mean our brain and body are doing nothing while we sleep.

During sleep many diverse physiological changes take place. The purpose of sleep isn’t yet fully understood; it has been hypothesized that, among other things, sleep allows both our brain and body to replenish and restore, as well as consolidating memory and strengthening the immune system. Lack of sleep, by contrast, has detrimental effects on many areas such as our physical health, cognitive abilities like memory and alertness, and emotional wellbeing.

According to the Sleep Homeostasis Hypothesis (SHY), sleep’s core function is to enable learning. SHY states that while we are awake and ‘experience’ things our brain cells form new connections with each other (this is called brain plasticity). Because of the skull there is a natural limit to these connections. However, during sleep when the brain is ‘offline’, some connections are cut back, allowing space for new learning to take place the next day. Clearly, many questions remain; how it is decided which connections to keep and which to cut back, for example. Luckily, researchers are working on this!

WHAT MAKES SLEEP HAPPEN?

Sleep happens when specific areas of our brain simultaneously activate and deactivate. Depending on which area is activated, either wakefulness or sleep will be promoted.

The arousal system is responsible for keeping us awake. This system is located within the brain and consists of certain parts of the hypothalamus, basal forebrain and brainstem. The sleep-promoting system also sits in the hypothalamus, which is involved in controlling many physiological factors and functions in your body. Using specific neurotransmitters (messenger molecules in the brain) and a mechanism not dissimilar to an electronic on–off switch (for which the scientific term is ‘flip-flop switch’), both systems inhibit one another. So if, for example, the arousal system is active it suppresses the sleep-promoting system. That’s what makes sleep and wakefulness mutually exclusive: you’re either asleep or awake, and the switch between the two states is normally rapid and complete. However, a malfunctioning switch — where the transitions are no longer swift and wakefulness spills over into sleep or vice versa — can be a cause for sleep disorders.

Neurotransmitters are used by the brain for cell-to-cell communication. Their interactions link different brain areas to form networks, which they then either activate or deactivate. Important excitatory neurotransmitters used by the arousal system include acetylcholine, orexin, serotonin and histamine (which explains why antihistamine tablets can make you drowsy), while those used by the sleep-promoting group include GABA and galanin.

Histamine, for example, makes you more alert, helps you to think more clearly and gets you motivated. GABA is the brain’s major inhibiting neurotransmitter. It basically blocks the effects of the excitatory neurotransmitters. So, sleep is of the brain and by the brain. But sleep isn’t just crucial for brain functioning; it’s also necessary for our body and mind to function optimally.

HOW IS SLEEP REGULATED?

Many of us see sleep and falling asleep like flipping a light switch: if I do this and don’t do that, I’ll sleep. But it’s not that straightforward. The cycle of being asleep and being awake is complex and involves several brain areas and signalling systems. Fundamentally, it’s the interaction of two somewhat opposing processes that regulates these brain regions and signals, and that regulates our sleep–wake behaviour.

One process monitors time awake and is responsible for the pressure we feel to sleep. We call this the sleep drive or sleep pressure. The second process is called the circadian pacemaker or internal body clock. This generates signals for sleep timing. It may appear a little complicated but a slow walk through the processes and a diagram can help. Take a look at Figure 1 and then let’s find out what each of the two processes actually are.

The sleep drive

The sleep drive, or the need to sleep (the thin black line in Figure 1), is affected by how long you have been awake: the longer this is, the greater the pressure to sleep. The sleep drive is often likened to an hourglass, where sand flows from one side to the other (from wakefulness to sleep), and when it is full it is flipped over and the sand flows back (from sleep to wakefulness).

To explain it in detail, the longer we’re awake, the more adenosine (a nucleoside) accumulates in our brain. Adenosine is a by-product of the brain’s metabolic processes and is seen as a biomarker of sleepiness because it binds specific groups of cells in the brain, slowing down their activity, which makes us feel tired. The more adenosine that is bound, the greater the increase in the drive to sleep. Once levels of bound adenosine hit a certain threshold — i.e. once one side of the hourglass is full — the likelihood of falling asleep is high. That’s when we feel really tired and generally go to bed and sleep. While we’re asleep, adenosine disconnects and the drive to sleep dissipates — the hourglass sand flows in the opposite direction.

Figure 1: The interaction of the sleep and circadian drives

In typical sleepers it takes about sixteen hours for adenosine levels to reach the necessary threshold, followed by an eight-hour sleep duration. But it only takes a few hours for the drive to sleep to dissipate. So if we were to wake up at this point, after approximately four hours’ sleep, we would have slept too little and this would have detrimental effects on our wellbeing and performance. Luckily the second mechanism, the circadian pacemaker, comes into play at this point.

The circadian pacemaker

The internal circadian clock (from the Latin circa meaning around, and dies meaning day) or internal body clock acts as our body’s timekeeping system. It’s responsible for setting the rhythm (timing and duration) of our behavioural, psychological and physiological functions and processes, one of them being the rhythm of sleep and wakefulness.

The circadian clock comprises a specific group of neurons called the suprachiasmatic nuclei (SCN), which sit in yet another area of the hypothalamus. I like to compare the SCN to the conductor of an orchestra, setting the rhythm for the rest of the body. This is important because each organ, and in fact most of our cells, has its own clock and would function according to its own rhythm if it wasn’t for the internal clock. Just as every musician in an orchestra has his or her own rhythm, without a conductor they soon would play out of time. For our bodies, the equivalent is that all our behavioural, physiological and psychological processes would be misaligned with one another as well as with the external day. We simply wouldn’t be able to function in a way appropriate to the time of day if it wasn’t for the master clock and its synchronizing abilities.

The internal clock has its own rhythm, which is slightly longer than the external 24-hour light/dark cycle; on average, it’s around 24 hours and eleven minutes. Over time this means our internal clock will start to lag behind the external day. When this happens, our individual activities and biological processes will be mistimed or get out of sync with the external day. As a result, we exhibit the ‘wrong’ behaviour for a given time of day and this can have the effect of reducing our chances of good health — and survival.

To better illustrate my point, here’s a short metaphor. Let’s go back in human evolution and imagine it’s several thousand years ago. You sleep during the day and you’re awake at night, which is when you leave the safety of your cave to go hunting. Unfortunately, your night vision isn’t great, so you don’t notice the lioness until your head is in her mouth … her night vision is far superior to yours.

Obviously, this isn’t an ideal state to be in. To prevent this from occurring, the clock needs to be synchronized by environmental time cues (known as zeitgeber, German for ‘time giver’) on a daily basis to be aligned with the solar 24-hour day. The 24-hour light/dark cycle is the strongest of these time cues; ‘lights on’ signals daytime and ‘lights off ’ signals night-time to the clock. Your internal clock then relays this information via the hormone melatonin to the rest of your body. (In Chapter 2 I’ll come back to this and explain how light impacts your internal clock and how it regulates the production of melatonin.)

The key point here is that your internal clock regulates when to sleep — and when not to sleep — during the 24-hour day. It maintains a separation of wakefulness and sleep, dividing them into different episodes with one during the day and the other at night, and perhaps a short one in the afternoon.

How does this help us to stay asleep?

How do the need to sleep and the internal clock interact?

In the early morning hours, when the sleep drive has dissipated and the ‘sleep’ side of the hourglass is empty, arousal from sleep becomes much more likely. At the same time, the circadian clock sends out a sleep-promoting signal to consolidate our sleep and prevent us from waking up too early. Once this signal ends, we wake up and the ‘sleep’ side of the hourglass starts to fill up again over the course of the day.

IS THERE SUCH A THING AS DEEP SLEEP?

Yes, there is. And there are even a number of other sleep stages, too. First of all, the brain shows distinct waves during wakefulness and sleep. Using polysomnography, where electrodes are attached to your head, researchers can record brain waves.

Sleep itself can further be divided into two broad states: rapid eye movement sleep (REM), characterized by fast, wake-like waves, and a quieter, non-rapid eye movement sleep (NREM). The repetition of alternate NREM and REM stages results in sleep cycles, each with a duration of 90 to 120 minutes. For the typical sleeper, this equates to four or five cycles per night. The graphical representation of this sleep-stage cycling is called a hypnogram — it’s the line that goes down and up in Figure 2 (I’m a great fan of visualizing things during a discussion). When I explain a hypnogram I often compare sleep to a symphony and the hypnogram to the score.

The squiggly lines on the left are the brain waves for each sleep stage. NREM sleep can be subdivided into three stages, each reflecting a different depth of sleep.

NREM stage 1

We enter sleep via NREM stage 1 (N1). This is a very light sleep: you become drowsy, your eyes move slowly and your muscles start to relax. Your brain waves also start to slow down and their oscillations become bigger — we call them theta waves. This is a transitional phase from wakefulness to sleep, when you’re neither fully here nor there, you’re simply gently drifting off to sleep. For example, if your partner says your name while you’re in this phase you’re likely to respond, and you can still detect certain types of smells. You may also experience sudden twitches or muscle spasms, called hypnic myoclonic or hypnic jerks. These are nothing to worry about. They’re just an involuntary twitch of a muscle — hypnic is short for hypnagogic, meaning that it’s happening during the transition from wake to sleep.

Figure 2: The hypnogram

You can also experience jerks at the transition from sleep to waking; these are called hypnopompic jerks. You might sense a feeling of falling or weightlessness and experience hallucinations or visual imagery at this point.

Since N1 is a lighter stage of sleep we can often misperceive such sleeping for being awake. For those lucky enough to have a bedfellow, have you ever turned to your partner to say, ‘I didn’t sleep last night’ only to have your partner look at you and say, ‘Yes, you did’? It may have been that during the night your sleep was disturbed and you spent more time in N1 than usual, and you interpreted your feeling of unrefreshed sleep as a lack of sleep altogether. However, if such feelings occur regularly then it’s important to see a sleep specialist to check for any potential sleep disorder, such as insomnia.

NREM stage 2

NREM stage 2 (N2) sleep is also a light phase of sleep, though one during which everything’s taken down a notch. It becomes more difficult to be woken up from N2 compared to N1. In addition to more theta waves, we see rapid brain wave features called sleep spindles and K-complexes appear. Both patterns play an important role in memory formation and learning. And on a physiological level, we observe a decrease in body temperature and a slowing down of breathing and heart rate.

Fascinating new studies have shown that, while in N2, the brain is still able to respond to external stimuli and perceptual learning occurs.

NREM stage 3

NREM stage 3 (N3) is what is called slow wave sleep (SWS) or ‘deep sl...