Do You Need a Brain to Sleep?

 

You’ve heard your parents say it, your co-workers say it, even the odd over-worked middle schooler say it: there’s just not enough hours in the day. If only we didn’t have to sleep, we could get so much more done! On average, people spend a third of their lives either asleep or trying to fall asleep. So why do we need to sleep as often as we do? While there is no one, definitive answer to this question, research suggests that sleep helps the brain with rejuvenation, restoration, and memory consolidation, among other things (as well as helping out with bodily functions outside of the brain). Your brain acts as the hub for most behaviors, including sleep. Simply put, it collects information and outputs a behavior in response to that data. So, when you spend energy during the day, your brain receives a signal that says, “it’s time to rest”. This is referred to the homeostatic drive to sleep. Your body needs a good balance, or homeostasis, of rest and activity, and your brain is responsible for returning you to that equilibrium. This is why, after a few days of very poor sleep, you may start to feel unnaturally drowsy during the day- if you have not been maintaining homeostasis, your brain looks for ways to catch you up. 

Whenever discussing sleep patterns and habits, it’s important to realize that your need to sleep is controlled by more than just a homeostatic drive. Your brain is an incredibly complicated machine, and so it has multiple ways of receiving information that says, “time to sleep”. Another input your brain relies on to judge when you need to sleep is referred to as a circadian rhythm. A circadian rhythm refers to any process that runs on a cyclical, repeating pattern, usually correlated with a 24-hour period. Even when we feel rested and don’t respond to the homeostatic drive, we still tend to get into bed at roughly the same time every night. At night, a lack of light inputs from our environment are telling our brain to shut off and go into rest, because light cues during the day are one of the factors that tell our brain to keep us awake. The most well-known rhythm is our sleep cycle, but lots of our body processes run on a circadian rhythm, such as body temperature or glucose levels. Incredibly, all of our internal circadian rhythms are synced- your lowest body temperature will also correlate with the middle of your sleeping state. This makes sense when you consider that there are certain areas of your brain that are tasked with running your internal clock, so all of your body’s rhythms are receiving timing information from the same source. 

Studying these internal rhythm mechanisms is integral to understanding sleep. Dr. Cavanaugh at Loyola University Chicago published a paper on his study looking at circadian outputs in fruit flies (Drosophila). Drosophila brains are excellent model organisms for studying sleep and rhythms in humans, because they have very similar internal clock mechanisms. His study, “A circadian output center controlling feeding: fasting rhythms in Drosophila”, looked at how the central clock in the brain regulates distinct circadian outputs, in this case specifically the feeding/fasting rhythm. By disrupting the central clock genes in the PI (homologous in humans with the hypothalamus), flies were observed to have disrupted feeding/fasting rhythms as well as rest/activity rhythms. 

With this information in mind, it would make sense to think that animals with a brain require sleep and experience sleep-like states, while organisms without brains (think: jellyfish, coral, or plankton) do not. However, a recent study looking at a kind of cnidarian (called a hydra) proposes the idea that sleep actually evolved before the brain. Hydras are tiny relatives of jellyfish that float through the ocean and are biologically immortal, and, notably, do not have brains or central nervous systems (rather, they possess a “diffuse network of nerves”). Still, they have been observed to cycle into sleep-like states, which are characterized by a decrease in movement and activity, even though these rest cycles do not follow a 24-hour period, like humans do. Scientists discovered that they could elicit sleep-like states in these organisms by exposing them to drugs (melatonin) and inhibitory neurotransmitters (GABA) that act as sleep-inducers in humans and drosophila. Given that the observed sleeping states are molecularly similar to the mechanisms of the cellular clocks in other organisms, the hydra study may also help to shed light on the origins of the molecular clocks that now reside in the brain, such as is seen in humans and drosophila. Hydras have a genome just like drosophila and humans, and this study identified proteins that can also be found in drosophila. Many drosophila studies (including Dr. Cavanaugh’s) have relied on the knowledge of genes that code for sleep mechanisms in the brain. By studying how sleep is enacted by the genome of a brainless organism, those findings may be applicable to future studies looking at circadian circuitry in the brain. Results from these studies have important implications in the health field for people who suffer from rhythm-related disorders, so it’s important to understand the origin of such complex behaviors.