For such a fundamental and routine feature of our existence, sleep is something that we know relatively little about. Though it has been studied since the 1900s, scientists still do not fully know why sleep is even strictly necessary, or which specific advantages have allowed this seemingly impractical behaviour to evolve. In a competitive environment, sleep could be expected to disadvantage an organism, detracting from the time available for decidedly more useful activities – eating or breeding for example – and leaving an individual vulnerable to attack by predators. Any potential benefit would, therefore, have had to outweigh these drawbacks for the habit to have survived the process of natural selection.
It appears that we are far from alone in this predicament. For all the richness and diversity of the animal kingdom, the need to sleep is a commonality that unites every species so far observed. Studies of the upside-down jellyfish (Cassiopea) suggests that even creatures with no centralised brain enter into a sleep-like state daily, and become sluggish and less responsive when this slumber is disrupted. Plants may not be exempt either. Findings from 2016 suggest that at night birch trees may enter a less-active state which could be compared to sleep, “drooping” their branches and leaves after the sun sets. Scientists believe that many other plant species could exhibit a similar pattern.
Gradually though, improvements in our understanding of the relationship between sleep and human physiology may be shedding light on this conundrum. Researchers from Massachusetts General Hospital have now discovered that sleep may be important for protecting our cardiovascular health. The team has uncovered a mechanism by which sleep might protect us from the buildup of fatty plaques in our blood vessels, a condition known as atherosclerosis. The group subjected a strain of mice prone to this disease to sleep deprivation and observed that the group of mice whose sleep was interrupted developed larger plaques than their undisturbed counterparts.
Further experiments suggested that this difference might be down to changes in a hormone called hypocretin, which was significantly reduced in the sleep-deprived mice. The evidence indicates that hypocretin normally controls the number of white blood cells produced, keeping them at normal levels. The group believes that due to the reduced amount of hypocretin in the sleep-deprived mice, many more white blood cells were circulating in their blood vessels. In atherosclerosis the presence of too many of these cells can often be problematic, as they contribute to inflammation and get caught up with the fatty deposits in the blood vessels, worsening the condition.
And it is not just cardiovascular disease that has been linked to sleep deprivation. A group at the National Institute on Alcohol Abuse and Alcoholism (NIAAA) recently found that prolonged disruption to sleep could increase the risk of beta-amyloid build-up, a protein associated with Alzheimer’s disease.
Despite the evident importance to health, the need to spend time asleep can still pose a problem for certain species, and we know that many animals have adapted means of minimising this disruption. Cetaceans, such as whales and dolphins, sleep with half their brain at a time, so one half can remain alert and reactive to threats. Bumblebees tending pupae nap half as much as they do normally – doubtless a relatable phenomenon for any new parent. During our evolutionary journey, humans may have also adapted to sleep less. A study by Dr. Charles Nunn at Duke University in California suggests that we sleep for fewer hours than other primates, possibly because of the extra time required for our complex social lives and for the development of a diverse range of skills.
The recent developments in our understanding of sleep and health are fascinating, however, there is still a great deal to learn. In the meantime, as beneficial as high-quality sleep can be it should be clarified that those Sunday morning lie-ins may not necessarily be good for your health either. “We know that, in terms of health, there is an optimal amount of sleep, and people who sleep more or less than that have health problems,” says Nunn. “It definitely seems that sleep has been optimised.”
Image: Paul Chiorean via Flickr