Sleep is a complex , very ordered series of events-yet it’s designed to happen naturally without us thinking about it, pretty much the same way that we breathe. If course, if you have a sleep problem, you become very aware of sleep and wakefulness, and it doesn’t seem so easy.
Whether you are a good sleeper or a poor one, it may be useful to find out a little bit about this mysterious thing called sleep. If you are feeling depressed , you will be particularly interested because sleep and depression are closely associated , as we shall see.
So what is sleep?
It is a state of greatly reduced awareness, taking up around one quarter to one third of our daily lives, during which the body repairs itself, and our memories, and emotions are consolidated . There is a lot of evidence that we need sleep for good mental functioning. It is, if you like, nutition for the brain – and we simply can’t do without it . When we are depressed our sleep is often upset , and this is a two-way street, because having insomnia also makes us more likely to get depressed in the first place.
When we measure sleep we look at three different aspects. We measure the brain’s electrical activity with an electroencephalogram (EEG), our musle activity with an electromyogram (EMG) and our eye movements with an electro – oculogram (EOG).
Together , these measurements are called polysomnography (PSG) ; a short-hand way of describing a full sleep assessment.
Stages of sleep
As we fall asleep we go into a transitional phase between wakefulness and sleep known as stage 1. Compared with quiet wakefulness , the EEG slows down , and your muscles begin to relax . If someone wakes yuo up during this very light first stage you might well say,’ Hey, I was just nodding off there!’ or you might startle.
After only a matter of minutes you progress on to stage 2 sleep. Here the EEG varies a lot with what we call ‘mixed frequencies ‘(some fast , some slow, some high amplitude , some low) . However, there are recognizable parts , called ‘K-complexes ‘ and ‘sleep spindles’. Different parts of this stage are important for our attention and memory . Stage 2 actually makes up about 60 per cent of our sleep , although when we first go into it we move quickly through it to the next stages.
Stage 3 and stage 4 arer deep sleep, or slow-wave sleep.We have our deepest sleep during the first couple of hours. I gues most of us would like the idea of having lots of deep sleep, but in fact it makes up only around 10-15 per cent, and less as we get older.Stages 3 and 4 sleep seem important for basic physical recovery growth and tissue repair.
We can see them that sleep involves not only a loss of consciousness , but also a steady change in the EEG. However, there is a form of sleep during which the eyeballs move rapidly (measured by EOG) , whereas the rest of the body is pretty much paralysed (measured by EMG) . The term ‘rapid eye movement ‘ (REM) sleep was coined to describe this.
We could easily injure ourselves by acting out our dreams, were it not for the factthat during REM sleep our muscles are ver relaxed . You may not have realised before that you are in fact ver still in your bed during your dreams, in spite of whatever vivid dream imagery you may experienced. Occasional muscles twitches are quite usual , but any movement on a large scale during REM sleep is uncommon . Although some people think that sleepwalking happens when we dream, in fact it is nothing to do with REM sleep. Sleepwalking and night terrorts happen in our deep sleep.
What about my personal experience sleep?
You have probably never to a sleep laboratory but I’m sure you’ve tried measure your sleep -perhaps by working out how long you think you have slept, how long it took you to fail asleep , or how many times you woke up . These are measures of the experience of sleep , of what you remember , and of the conclusions that you draw. It isn’t easy to calculate these things very accurately!
You may even have tried to keep a sleep diary so that you can see what your sleep is like over a period of time, to work out if there is a pattern.
Diaries like this can be useful and I will show you one shortly.
Sometimes it is easier to think about the quality of our sleep, rather than its quantity . Good sleep – quality might be when feel that you have had a ” great sleep” or ” hardly any sleep” or that it took you a ” long time to get into a proper sleep” . Whether we are trying to estimate quantity or commenting on quality , this is called ” subjective assessment” . We should not fail into the trap of thinking that subjective assessment is less important than objective assessment like PSG; ( the measures described above). What we think and feel about our sleep is terribly important , not least because it is our experience of it ( or lack of it) that relates most closely to the complaint of poor sleep.
Interestingly, when we compare people ‘s judgement of their own sleep with more objective measures, we see that they usually sleep longer than they think they have done. This tells us that people tend to overestimate how long it has taken to fall asleep, and how long they have been awake during the night , and to underestimate the total amount of sleep that they have had . It’s not just people with insomnia who exaggerate thgeir sleep complaint. People who are normally good sleepers also make very similar errors in estimation on those occasional nights when they sleep oorly.
The problem in fact, may also be with the PSG, since it does not identify subtle EEG characteristics that form part of the underlying pattern in insomnia .
For example, the PSG may not always pick up the frequent micro-arousals that intrude into sleep( particularly common in insomnia ) which can give us the subjective impression that we are awake.
What controls our sleep pattern?
Two processes work together to regulate our sleep pattern. One is called THE SLEEP HOMEOSTAT , and this controls ‘drive’ for sleep; the other is called THE CIRCADIAN TIMER , and this controls when we sleep.
Broadly speaking , the longer we are awake, the sleepier we become . Sleep reduces the drive for sleep, and wakefulness increases it, much in the same way as we become parched if we go without fluid , and thirst increases . Drinking satisfies that thirst and so reduces the drive to drink.
With each our that we spend awake we accumulate sleep debt. In healthy good sleepers this debt is rapid by the night’s sleep and they awaken refreshed and ‘in balance’ the next morning . Pressure for sleep is stronger at the start of a sleep episode than it is later on and this accounts, for example , for why a nap can make us feel much better . Similarly, some people report feeling quite refreshed after a couple of hours of sleep. The restorative value of these early cycles of sleep apperas to be proportinally greater.
Of course, we might be advised to avoid napping because it has the potential to reduce the drive for sleep at night.
The second sleep process concerns our circadian rhythms. The word ‘circadian’ derives the Latin words circa diem, literally meaning ‘around the day’. Sometimes we talk about the ‘body clock’, meaning pretty much the same thing . The term is used to describe the harmony of the sleep-wake schedule. Other biological functions such as body temperature also follow recognised circadian patterns. We are designed to function in a twenty-four-hour world! This may sound like science fiction , but it is science fact, which is really much more exciting and interesting.
The hormone melatonin is largely responsible for the regulation of the bodyclock throught our lives. Melatonin is produced in the brain , in the pinal gland. Its production rate is dictated by natural light, so that during darkness (the normal sleep period) melatonin production increases , and with the coming of daylight , it is shut down. Of course there is natural variation in circadian alertness during the day. You will probably be aware of ‘the afternoon dip’ when we feel temporarily more tired. Indeed , in some societies it is normal to have a siesta at this time, because it also coincides with the hottest part of the day. In terms of our circadian tendencies there is something to be said for that lifestyle!
It is important to note that it is the interaction of the sleep drive and the circadian timing mechanism that, under normal circumstances , leads to good sleep. This is when the drive for sleep becomes strongest during normal hourds of darkness, and the results in an absence of pressure for sleep during wakeful , daylight hours.
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