Sleep still is one of the greatest biological mysteries. After many decades of research, we know all about what causes us to feel sleepy, different types of sleep, electrical activity in our brains while we sleep, and the mechanisms of waking up after sleep, but we still don’t know why we sleep.
It is clear we do need regular and normal sleep for our overall health. All animals have some form of sleep, so far as researchers have been able to test. Sleep is strictly regulated. Sleep loss is inevitably followed by a rebound.
Not sleeping leads to unstable moods, impaired cognition, and eventually death. Sleep deprivation elevates the body’s production of stress hormones, raises blood pressure and boosts blood levels of substances that cause inflammation. It is linked to serious diseases such as heart disease, psychosis, anxiety disorders and depression.
Recent research even showed that people, who don’t sleep enough, become more obese. Sleep deprivation may disrupt hormones that regulate glucose metabolism and appetite. Less sleep results in higher levels of ghrelin, an appetite-stimulating hormone, and lower levels of leptin, an appetite-suppressing hormone.
Apparently, some benefits emerge from the environmental disconnection sleep produces that cannot be provided by quiet wakefulness. Several theories have been proposed. Sleep might be to save energy, but the facts prove otherwise. The amount of energy used during sleep is just a bit less from the amount necessary during wakefulness. Hibernation is an excellent example of a way to save energy. But animals come out of hibernation at regular intervals to fall asleep, which indicates sleep has another function than saving energy.
The immune system surely is positively influenced by sleep. Research among mammalian species came up with a higher number of white blood cells, the building blocks of the immune system, the more total hours of sleep a species enjoys. Sleep deprivation in rats showed a twenty percent decrease in their number of white blood cells.
On a cellular basis, sleep may be a behaviour for recovery and return to homeostasis. During wakefulness, many metabolic processes are pushed to their maximum levels. Most of the brain and peripheral organs appear to be recovering during sleep.
Sleep may also be essential for the brain’s plasticity. During the day, contacts between neurons increase in strength. Sleep may be used to damp down to baseline levels. Receptors get a ‘rest’ and regain their sensitivity. If this process is prevented by sleep deprivation, symptoms occur related to an overload of brain circuits.
The relationship between memory and sleep is one of the most evident. Many studies have shown that while sleeping the brain is busy consolidating memories. The neurons in rat brains firing while learning a new task, are active exactly the same way during the following sleep period, as if to imprint the path necessary for performing the task. Rats allowed to sleep after learning, perform significantly better thereafter than their colleagues who were prevented from sleeping. A study among adolescents also reported better academic performance with more high-quality sleep, whereas going to bed late over the weekend worsened academic performance.
The most fascinating theory about memory and sleep hints at the existence of an intermediate memory system next to the short-term and long- term memory systems. According to this theory, memories aren’t transferred from short-term storage to long-term storage during waking time, but go into temporary memory storage. The function of sleep would be to process, encode, and transfer data from this temporary to long-term memory.
To ensure an uninterrupted memory transfer process it is necessary to shut off temporary memory from the environment. Hence the urge to sleep. Sleep deprivation fills up the temporary memory store, which hampers cognitive skills. Sleep rebound is necessary to empty temporary memory and therefore takes up far less time than the actual time of sleep loss.
That we forget most of our dreams and sometimes only remember short fragments of dreams can be explained by the fact that intermediate memory isn’t available for storage during sleep and only short-term memory is taking in new information. It would be an interesting challenge to look for the neural circuits involved in intermediate memory and thus test this theory of sleep by reality.
An intriguing link exists between sleep and depression. It is evident depressed people don’t sleep well. But sleep and the neural processes going on in the brain during sleep can actually cause depression. Several hormones and neurotransmitters that play a role in depression are also important in sleep processes.
Melatonin is a hormone produced by the pineal gland. It governs the entire sleep/wake cycle. The neurotransmitter norepinephrine, also pinpointed in depression disorders, regulates the activity of the pineal gland. Binding of norepinephrine to its receptors in the pineal gland triggers a cascade of messenger substances. Among these messengers is cyclic adenosine monophosphate (cAMP), which contributes to the synthesis of serotonin – another suspect in depression disorders. Serotonin in turn is used to synthesise melatonin.
The levels of melatonin in the brain vary in a daily cycle. Its secretion peaks in the middle of the night. The production of melatonin is inhibited by light. A specific type of depression, seasonal affective disorder (SAD), is caused by a deficiency of the system that produces melatonin. Fooled by winter’s darkness, the production of melatonin continues. This results in an inability to shake off the night’s slumber.
Sleep deprivation can alleviate symptoms of depression, especially if Rapid Eye Movement (REM) sleep, one of the phases of our total sleep cycle, is prevented. During REM-sleep, levels of serotonin in the brain drop. If, due to sleep deprivation, this natural decrease in serotonin doesn’t occur, serotonin levels – that are low in depressed persons – are elevated. Unfortunately, symptoms return if the patient’s sleep cycle is back to normal. Treatment of depression with tricyclic antidepressants happens to suppress REM sleep.
Another hormonal alteration during depression involves cortisol, produced by the adrenal gland. Cortisol is also referred to as the stress hormone as it is involved in the body’s response to stress and anxiety. Depression is characterised by raised levels of cortisol, but levels of cortisol normally also rise gradually in the course of the night.
Clear evidence exists for a strong link between mood and sleep, not in the least because in most cases, symptoms of depression are felt most intensely in the morning after waking up. Individuals with mild episodic depression and seasonal affective disorder have lower than normal melatonin levels. People who suffer major depression or panic disorder also have low levels of melatonin, as research has shown. Administration of melatonin, which possibly causes a surge in serotonin levels, may help alleviate the symptoms of depression. It may also improve sleep patterns in patients with depression. Thus melatonin deserves a more central role in future sleep and depression research.
In the end, it may even turn out it’s not the depression that causes patients to sleep badly, but disturbed sleep patterns that cause the depression.
Waste of time
Unfortunately, sleeping is not one of the favoured activities in our present society. Sleep is often seen as a waste of time. People who need little sleep (or at least claim that they do) are being admired. In television series about hospitals and emergency rooms, the sleepless nights of the resident physicians are being romanticised, whereas in reality a lack of sleep can lead to serious mistakes with dramatic consequences.
Psychology professor and neuropsychological researcher Stanley Coren, author of the book Sleep Thieves, an eye-opening exploration into the science and mysteries of sleep, puts the problem into words:
“Human beings seem to be fighting the evolutionary pressure to sleep. Despite the fact that sleep plays a vital part in our health and our efficiency, we seem out to abolish sleep. The first step in this process was the invention of the electric light bulb, which eliminated our main excuse for stopping our work at the end of the day, namely, that it was too dark to function. Next came the continuous conveyor belt, which encouraged factories to operate 24 hours a day. Now the continuous access to information provided by the internet and other computer communication links keeps us from our beds at all hours of the night. The work ethic we have adopted today says we should do away with sleep, or at least eliminate as much sleep time as possible. The movers and shakers of the world don’t waste their time sleeping. Yet too little sleep can kill us outright or can cause a gradual deterioration in our health. Too little sleep can make us clumsy, stupid, and accident-prone. Too little sleep can destroy our psychological motivation and put us into a deep depression.”
Sunset © rasica – Fotolia.com
Jumping sheep © julien tromeur – Fotolia.com
Waste of time © jesadaphorn – Fotolia.com