We've all been there. It's 2am and you are lying in bed, with a big project or a big decision on your mind. With a big day to come, you have to get a decent night's rest. Despite you knowing this, you're still wide awake. You try different strategies for relaxing. Deep breathing, imagining restful mountain scenery, but nothing seems to work. This is all typical speciocentric human thinking. So, if you want a good night's sleep, you need to think more like a zebra or a walrus.
For the vast majority of species on this planet, the most upsetting things in life are acute physical crises. Imagine you are that zebra and a lion has just leaped out and ripped open your stomach. While you've managed to get away, you now need to spend the next hour or so evading the lion as it stalks you. But, a zebra's body physiological response mechanisms are incredibly adapted for dealing with a short-term physical emergency like this, after which it's over with you or you're over with it.
Humans react differently. They lie around and worry about stressful things like mortgages and work, turning on the same physiological responses that gear us up to fight or flee. This is not an ideal state of mind or body to be, especially when you want to get some sleep. So what can you do? A great place to start is to understand your brain when it sleeps.
Sleep is not a monolithic process. There are different types of sleep: shallow sleep, in stages one and two, during which you are easily awakened. After which you have the deep sleep, stages three and four, also known as a slow-wave sleep. During REM sleep, your eyes dart around and dreams happen. Your sleep starts off shallow and you gradually sleep your way down to slow-wave sleep, followed by REM, then back up again. Then repeat the whole cycle, about every 90 minutes.
The brain works differently in different stages of sleep, as indicated in studies whereby people were monitored with a brain scanner, while you measure the levels of activity of different brain regions. During slow-wave sleep, parts of the brain associated with arousal activity slow down. As do brain regions involved in controlling muscle movement. The areas of the brain that first respond to sensory information have somewhat of a metabolic shutdown, which creates a metabolically quiescent sleeping brain. But, a very different picture emerges during REM sleep. Overall, there's an increase in activity. Some brain regions become even more metabolically active than they are when you are awake. There is also increased activity in the limbic system, which is involved in emotion. This is how sleep generally works. But what happens when you are stressed?
As we enter into slow-wave sleep, the sympathetic nervous system keeps us 'wired' relinquishing control to the parasympathetic nervous system, producing a calm, vegetative state. This calming effect is reinforced by a decrease in levels of glucocorticoid, or brain fuel. As you're mobilizing energy to generate your dream imagery your eyes move rapidly during REM sleep. At which point glucocorticoid secretion and the sympathetic nervous system rev up again. But because that slow-wave stage makes up most of what counts as a good night's rest, sleep is predominantly a time when the stress response is turned off. Then about an hour before you wake up, levels of certain 'wake up hormones and glucocorticoids begin to rise. This is not because rising stress-hormone levels play a role in terminating sleep.
When you deprive yourself of sleep, the sleep-induced decline in the levels of those stress hormones has no chance to occur. Rather, what happens is that glucocorticoids levels increase and the sympathetic nervous system is activated. During sleep deprivation, the elevated glucocorticoid levels play a role in breaking down some of the stored forms of energy in the brain which could have something to do with why learning and memory are so lousy when you're sleep-deprived.
In a recent study, they tested one way in which our brains become impaired when we try to think hard after we haven't slept. The study conducted tests on a rested subject, using a brain imager, and asked her to add sequences of three-digit numbers, at which point her frontal cortex lights up metabolically. But, when someone was sleep deprived and was given the same math exercise, they aren't as able to solve the problem. And in this case, the frontal cortex appeared to be too groggy to compute. In fact, the opposite occurs: the frontal cortex is activated but so are large parts of the rest of the cortex.
Of course, we're accustomed to all sorts of amenities in our modern lives. Some people must also work under conditions of sleep deprivation, this includes nurses and round-the-clock technical-support staff. But because we are a nocturnal species, if a person works at night or works in shifts, regardless of how many hours of sleep they get, it will still go against their biological nature. When working these sorts of hours, it tends to over-activate the stress response. In fact, it should come as no surprise that night work or shift work increases the risk of cardiovascular disease, gastrointestinal disorders immune suppression and fertility problems.
Sleeping habits have changed. In 1910, the average American slept nine hours a night. We now average 7.5, and that's declining.
What happens to sleep during stress? The hormone CRH seems to be responsible. It starts the glucocorticoid cascade by stimulating the release of another hormone called ACTH from the pituitary. But it's also the neurotransmitter that activates fear, anxiety, and arousal pathways in the brain. Unsurprisingly, about three out of four cases of insomnia are triggered by a major stressor. Studies have also concluded that poor sleepers tend to have higher levels of sympathetic arousal or of glucocorticoids in their bloodstream.
Maximum stress can do more than minimize sleep. It can compromise the quality of the sleep you manage to get. CRH infusion, for instance, decreases the total amount of sleep by decreasing slow-wave sleep, exactly the type you need for energy restoration. Rather, your sleep cycle is dominated by shallow-sleep stages.
This means that you wake up more easily-fragmented sleep. Even the slow-wave sleep you do get could be disrupted. Ideal slow-wave sleep shows a characteristic pattern in what is called the 'delta power range' which can be detected on an electroencephalogram (EEG) recording. If you are stressed before sleeping, or you are infused with glucocorticoids during sleep, you get less of that helpful sleep pattern during slow-wave sleep.
This can cause some real problems, as a lack of sleep, or poor-quality sleep activates the stress response and an activated stress response makes for less sleep or lower-quality sleep. Each feeds on the other. In one study, it was suggested that the expectation that you're going to sleep poorly makes you stressed enough to get poor-quality sleep. In the study, one group of volunteers was allowed to sleep for as long as they wanted, which was till around 9 am. But as expected, their stress-hormone levels began to rise at around 8 am. Basically, it would appear that they had enough sleep by about this time and their brains, felt happily restored and re-energized, and knew it. They, therefore, started secreting those stress hormones to prepare to end the sleep.
The second group of volunteers went to sleep at the same time as the first but were told that they would be woken up at 6 am. So, at 5 am, their stress-hormone levels also began to rise. What's important to note here is whether their stress hormones kicked in three hours earlier than those of the other group because they needed three fewer hours of sleep? The answer is no.
The rise was due to stressfulness of anticipating being woken up earlier than desirable. Their brains were feeling that anticipatory stress while sleeping. This demonstrates that a sleeping brain is still a working brain. Too little, continuous, uninterrupted sleep, going to sleep late and getting up early is not good. What's worse? too little sleep that is unpredictably fragmented. You go to sleep with the corrosive knowledge that 5 hours or 5 minutes from now the alarm will go off.
This teaches us a lot about what counts as good sleep and how stress can prevent it. When it comes to what causes physiological stress, a lack of predictability and control in your life are the top things you want to avoid.