Fruit flies do it. Dolphins do it. Even round worms do it.
Yes, you guessed it, I’m talking about sleep.
Sleep serves multiple functions, some of which are still not understood. To determine what goes on during sleep, and why it is so crucial for every species from the fruit fly to the jellyfish, a trio of Penn experts recently published a piece in Nature Reviews Neuroscience looking at sleep function across phylogeny -- that is, the evolutionary development of species -- to find the origins of the need for sleep.
We know that sleep plays a critical role in the evolution of many species – just think about how often an infant needs to sleep to grow and develop – but what is still not clear is whether the original purpose for sleep remains the same today.
Drawing from previous work in fruit flies – which, like humans, have a central nervous system and a brain, and sleep and wake periods that follow a 24-hour circadian rhythm – the authors speculate that the functions of normal, healthy sleep, and those of recovery sleep (following sleep deprivation) are very different.
Recent sleep deprivation studies have shown that slumber benefits memory formation, brain health, the immune system, and even skin. Although this work informs our understanding of the importance of healthy sleep and efforts to improve public health, the team argues that those studies do not necessarily explain the normal functions of sleep.
For example, recovery sleep engages different physiological processes than those regulating normal, healthy sleep-wake cycles. Normal, healthy sleep, and recovery sleep are controlled by different genes. Additionally, the neural circuits that function during natural sleep are different from those that are active during sleep when recovering from prior sleep deprivation.
“The field needed well-designed research into healthy sleep across the evolution of various species to find what the original point of sleep really is,” said David Raizen, MD, an associate professor of Neurology and senior author on the research. “The debate about the function of sleep continues – but is now informed by remarkable findings in the expanding world of sleep and chronobiology research in multiple species.”
So what do we know?
Well, sleeping habits differ considerably from species to species. For example, in dolphins, half of the brain stays alert (to tell the dolphin to breathe and be alert for possible danger), while the other half is asleep. Research in jellyfish, on the other hand, has shown that neither a brain, nor a central nervous system, is needed for sleep. And, unlike mammals, who rely heavily on the rapid eye movement (REM) stage of sleep (the stage in which most dreaming happens), insects and fish don’t appear to need REM sleep at all. Mammals and birds experience REM and Non-REM sleep. Non-REM sleep consists of three sleep stages, with the body experiencing the most restorative deep sleep in the third stage. Researchers also suspect that REM and Non-REM sleep are also present in reptiles, which could mean that this sleep was present as early as 300 million years ago!
Sleeping duration also varies significantly. For example, each day, brown bats sleep 20 hours, while plant-eating animals sleep two to three hours.
Although duration and behavior varies, the function of sleep to aid in metabolism, development, and brain health is shared by distant species. This commonality in purpose, even though it manifests itself in vastly different ways, suggests a common origin for sleep early in evolution.
For example, when cockroaches (the first invertebrates observed to sleep) are forced to continue moving during their regular sleeping time, they become even less mobile, showing that they are protecting their sleep behavior; scientists term this behavior “sleep homeostasis’’. Similar behavior has been seen in honey bees, fruit flies, and even round worms.
With all that’s known about the purpose and practice of sleep, even today, new discoveries are achieved, and new questions surface.
In recent years, a growing body of research has shown that disrupting sleep-wake cycles and eating outside of the “normal” schedule (i.e., skipping breakfast, eating a late dinner, and snacking in the wee hours of the morning) disrupts the body’s natural circadian (24-hour) rhythm, the cycle that tells us when to sleep, wake up, and eat, and affects hormone levels and other functions. Research by Penn experts in Sleep and Psychiatry, is underway to help us understand the mechanisms that explain these effects in different populations of patients.
Just this week, Penn team led by Dr. Amita Sehgal published research in Science describing the discovery of a gene called nemuri, which increases sleep drive in fruit flies. The same gene is also active when the body is fighting infection and works to kill sickness-causing microbes. Now, those researchers are looking into how this gene drives both sleep and immunity.
Studies have shown that sleep flushes out beta-amyloid toxins in brain that are linked to Alzheimer’s disease. In an AlzForum article, Sigrid C. Veasey, MD, a professor of Sleep Medicine, said that levels of tau – a protein implicated in Alzheimer’s -- appear to vary more broadly among sleep-deprived individuals compared to those sleeping normally. The findings, she said, suggest that some individuals may be more at risk of this negative effect.
Looking at sleep during the evolution of other species is expected to continue yield new opportunities to advance our understanding about why living organisms sleep.
The moral of the (bedtime) story?
Although we may not quite know exactly why just yet, sleep is important across all animal species, and at all ages. Good night.