The problem with this kind of question is that it doesn't propose any kind of causal path between one concept and another, leading to a lot of chicken-and-egg problems. The concepts may indeed be related, but the theoretical model that the researcher wants to test with his or her study is an undifferentiated mess. A research question that says "are there complex inter-relationships between A, B, and C?" can almost always be answered simply with "yes." In life, almost everything is related to everything else, and human behavior in particular is the product of many weak forces. But the questions of how and in what order things are related are the truly interesting ones. That kind of question shows you the linchpins in a process, the joints where one might be able to exert a bit of pressure and get the whole system to move in response.
The question of causal pathways came up in a recent discussion that I was having with some medical students. I was speaking about my own experience of brain injury after a car accident in 2016, which I have written about previously. After talking about my problem with balance, I mentioned in passing that I could often tell I was off-balance because I started waking up in the middle of the night. One of the students asked about the causal path: what does balance have to do with sleep? I confess that I hadn't thought about this before -- I simply knew from my experience that sleep problems happen soon after I start to have balance problems, but before I start to get more serious symptoms like exhaustion, mood changes, or trouble thinking. They serve as a red flag and enable me to take steps for prevention. In this example, the causal order comes from my own trial-and-error experience, and for me it quite clearly looks like this:
balance -> sleep -> mood, thinking, fatigue, etc.
To answer my student's question, I started digging into the literature, and I immediately discovered that other causal orders are also possible for the same set of symptom constructs. For instance, I have written before about the pervasive health-related consequences of disrupted sleep. Sleep problems are one of the most common triggers for a wealth of symptoms, including the well-known mood-sleep-fatigue-pain symptom cluster in cancer. Poor sleep contributes to inflammation, which is associated with sickness behavior, tiredness, and depressed mood, as well as increased pain sensitivity. Sleep problems were also associated with an increased incidence of vertigo in a nationwide sample. Inflammation, which is known to be associated with poor sleep, is a logical mechanism of action for this: swelling of tissue either in the inner ear or around the vestibular nerve could cause people to feel dizzy. But this causal path has the following form, with sleep as the starting point:
sleep -> balance -> mood, thinking, fatigue, etc., or maybe just
sleep -> inflammation -> balance and everything else
That's logical, and fits with current biological understandings of symptom clusters associated with sleep, but it just doesn't fit my experience. I can clearly note balance problems (when I happen to be looking for them) before the sleep problems begin. So either I am wrong about my experience, or a different causal path is play.
The literature suggests another causal path, starting from depression. The most common symptoms of depression include not just sadness, but also loss of energy (i.e., fatigue), trouble concentrating, and sleep problems. Sleep problems might then lead to balance problems, as noted above. Or the low energy associated with depression might lead someone to stop exercising, which could in turn lead to balance problems. Evidence for this idea comes from the treatment literature, which shows that specific types of exercise can reduce dizziness -- as was in fact the case for me when I finally got a handle on my balance problems. If I feel depressed, maybe I stop exercising, and my balance problems return. Here is a causal path that prioritizes mood as the starting point:
depressed mood -> sleep and/or exercise -> balance -> thinking, fatigue, etc.
Again, this is a causal path with plenty of empirical support, and it makes logical sense. But unfortunately it doesn't fit with my experience!
In what causal model would it make sense for balance problems to come first? My first clue came from reports of "positional vertigo," a condition in which people feel dizzy only when their head is held in a particular position. Usually this is the result of crystals forming in the inner ear: Head motion shifts the crystals, the delicate inner-ear systems react, and the person feels a sense of movement out of line with their actual body position. The connection to sleep is that people's heads shift position when lying down, producing frequent vertigo at bedtime. Disrupted sleep can then lead to a range of other difficulties. This causal path looks a lot closer to mine:
ear crystals/head position -> balance -> sleep problems -> mood, thinking, fatigue, etc.
One problem, though: I don't have crystals in my ears. The problem as I understand it is a disconnect between the positional signals from my eyes and the sense of position from my body (interoception), with my eyes being more often mistaken. Over time, and especially when I tax the system with stressors like ladders or long drives, trying to bring those two data feeds into alignment wears me out.
A more neurological path for the route from balance to sleep goes through the suprachasmic nucleus (SCN) of the brain. The SCN regulates sleep and wake cycles, but as I have written previously, it relies on external cues known as "zeitgebers" (time-givers) to stay in sync with its surroundings. Left to its own devices, the brain runs on a cycle that's slightly longer than 24 hours. Some of the cues that the brain uses to reset its daily internal clock are light, temperature, and maybe also the sensation of lying down. When the brain gets confused about its position relative to the ground, that might also put its internal neural clock out of time with the cycle of day and night. This possible causal theory suggests an interesting hypothesis: Astronauts, who are completely separated from gravity and have no physical sense of up or down, might also have sleep problems. When I looked into this idea, I found that astronauts do in fact have trouble sleeping, and even more strikingly they report that sleep is difficult because they lack the sensation of lying down. The same effect can be produced here on earth by immersing people in water for 24 hours or more. Additional evidence comes from studies of balance exercises like tai chi, which have shown beneficial effects on sleep. Interestingly, the same study showed cognitive benefits of tai chi as well. Here's the revised causal pathway:
balance -> gravity-related zeitgebers -> sleep -> mood, thinking, fatigue, etc.
Now, it might be the case that mood, thinking, fatigue, and other changes are indirectly caused by poor sleep, or it might be that balance also affects other areas of the brain that more directly produce those effects. But this sequence at last matches the order in which things occur in my personal experience. My new hypothesis, then, is that just like astronauts I experience a sort of weightlessness when I am off-balance at bedtime, and that this leads to disrupted sleep and other emotional or behavioral problems. I might further hypothesize that the balance centers of my brain (the cerebellum at the back of the head) are feeding wrong information to the SCN, or maybe even (given the role of vision in producing my symptoms) that the signals from my cerebellum and my ocular nerve conflict. These are very individualized diagnoses, but they might suggest specific areas of damage that could be viewed in an MRI scan or seen in other patients who had similar head injuries. We might be able to devise some clever diagnostic test that differentiates my kind of balance-sleep-mood connection from someone else's mood-sleep-balance problem, or a third person's sleep-mood-balance concerns.
You can see from these examples just how important it is to establish the correct links and sequence of items in a causal chain. If sleep itself was the starting point, then it might make sense to reduce caffeine intake, purchase extra-heavy curtains to block out light, or take sleep medication (a strategy that a neurologist did, in fact, try without success early in my own treatment). If mood was the starting point, then antidepressants or cognitive-behavioral therapy might be the treatment of choice (again, things that didn't help me). If the problem originated in my ears, then an otolaryngologist might need to be consulted. But when the chain starts with balance, then physical therapy exercises focused on balance are likely to be the most effective form of treatment, and that's what has been true for me.
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