We all know the importance of sleep. We know how terrible it feels to be deprived of sleep for just a single night. Even relatively minor losses in sleep quantity or quality can affect mood and cognitive function the next day, especially if a person has been losing sleep night in and night out.
The latter process, known as accruing “sleep debt,” isn’t just a nuisance. The Centers for Disease Control and Prevention have gone so far as to label insufficient sleep an “epidemic,” after a survey of over 74,000 U.S. adults found 38% reported unintentionally falling asleep at least once in the past month, and 5% said they’d nodded off while driving a car.
But knowing that sleep is important doesn’t mean we know the importance of sleep, just as knowing that plants die without sunlight doesn’t require you to know anything about photosynthesis. Explaining just what sleep is and why we have to do it has actually proven quite difficult.
Surprisingly, sleep is hard to define concretely despite its existence across the animal kingdom. Insects, amphibians, reptiles and mammals all show signs of sleeping (sorry sponges, not you).
But understanding different species’ slumber and how it compares to ours can be challenging. What do we make of animals that show no signs of sleep deprivation (tilapia)? That sleep just three hours a day (horses), or don’t sleep at all (bullfrogs)? Or those that put just one half of their brain to sleep at a time (dolphins, seals, chickens and more)?
This diversity is huge. Consider eyes. No matter what animal they belong to, a set of eyes is always detecting the light bouncing off things. Blood vessels always carry oxygen. Wings are always used for flying, with rare exception (sorry sagehens, not you).
Describing what sleep ‘always’ does is nearly impossible, especially considering how subjective sleep is compared to say, kidney function.
This difficulty is due in part to sleep’s lengthy evolutionary history. Recent research suggests the molecular mechanism for controlling arousal in animals has been around for 700 million years, older than dinosaurs, mammals and terrestrial plant life. That means the behavior has had plenty of time to adapt widely different functions.
Hypotheses about the universal underpinnings of why animals must sleep have thus remained elusive. One of the best conjectures comes from Jerome Siegel of the UCLA Department of Psychiatry, who believes the ‘core function’ of sleep is simple: keep yourself alive when there’s nothing else to do.
Consider animals that are diurnal (active during the day). The senses these animals possess are all adapted to work best during the day. The animals they eat are all diurnal as well. Everything they do is designed to happen under the sun.
This evolutionary specialization makes being up and about at night a dangerous prospect. Diurnal species typically rely heavily on sight, which as you may know, doesn’t work very well at night. Avoiding predators and finding food is extremely difficult for diurnal animals at night—why not just pass out in a hole for a few hours and get up when it’s light again?
For Siegel, all the other functions we typically associate with sleep, like mental and physical rest, are à la carte items that evolution ordered just for us, as well as all the animals whose specific ecological niche require these functions. Sleep is something of a utility function serving a particular purpose for different animals depending on their needs.
But not everyone is on board with Siegel’s theory, however. Giulio Tononi and Chiara Cirelli of the University of Wisconsin believes that sleep plays a more fundamental role in the body. Their hypothesis is that sleep provides “synaptic homeostasis.”
Basically, they argue that throughout the day, we are continuously strengthening connections between neurons throughout the brain. It’s the mechanism by which we learn new things, and essentially every conscious second can be thought of as a process of learning.
But Tononi and Cirelli note that this can’t go on forever. As learning is coded as a relative change in synapse strength, learning would become impossible if we continue to make stronger and stronger synapses, since relative differences in synaptic strength would become too small.
What is our brain’s answer? A process called synaptic pruning, in which weaker connections made during the day fade away and stronger ones remain. It isn’t yet clear whether synaptic homeostasis is merely one of the adapted functions of sleep in animals with highly malleable brains or if it represents a true “core function” of sleep.
What’s amazing is how, subjectively, sleep is easy to describe—it’s that time when we’re paralyzed and unresponsive to the outside world, and our experience is limited to mental projections over which we seem to wield no control. It takes up about a third of our life, give or take, and many of us spend our days sipping coffee just to ward off its effects.
Objectively, sleep remains confusing and mysterious, a tenuous concept that requires further contemplation—especially in those quiet times just before we fall into its arms.
Warren Szewczyk PO ‘15 is hoping to spend the next couple years researching schizophrenia and continuing to write about science. If you are reading this, you hopefully read the entire preceding column, so thanks for that.