People who maintain a healthy lifestyle will have spent nearly one-third of their lives sleeping, yet researchers still do not fully understand the processes that drive sleep patterns. Every species experiences this fundamental activity, which is characterized by increased sensory thresholds, decreased muscle tone and movement, and—of course—a loss of consciousness. During a minisymposium at the 2017 SIAM Annual Meeting, taking place in Pittsburgh, Pa. this week, Alicia Prieto Langarica of Youngstown State University modeled the effects of temperature on rapid eye movement (REM) and non-REM sleep dynamics.
"Across species, all animals present some form of sleep," Langarica said. "Until very recently, nobody knew why. If you’re tired and you sleep, why isn’t it the same as laying down and closing your eyes? Why is a loss of consciousness necessary?” Given that sleep comes with distinct disadvantages, such as vulnerability to predators and limited time for resource acquisition, Langarica notes that the advantages must certainly outweigh the costs.
Brain activity under the following stages of wakefulness: fully awake, calm wakefulness, non-REM stages 1 and 2, deeper stages 3 and 4, and REM sleep. The patterns of REM sleep and wakefulness look remarkably similar. Image credit: Alicia Prieto Langarica, AN17 presentation.
Sleep patterns also tend to be non-uniform, as one typically experiences four to five phases of REM sleep and non-REM sleep during the night. REM sleep is characterized by increased dreaming and elevated breathing and pulse. It usually occurs at 90-120 minute intervals; the first bout is quite short, and the cycles elongate throughout the night. When waking naturally—without the aid of an alarm clock—one always wakes from a REM cycle. Langarica showed the audience a visual of brain activity under the following stages of wakefulness: fully awake, calm wakefulness, non-REM stages 1 and 2, deeper stages 3 and 4, and REM sleep. She noted the dramatic delta waves of stages 3 and 4, which signify deep sleep. “People are obsessed with getting a lot of this,” Langarica said of deep sleep. “But if you get a lot of this, you’re in a coma.” She also pointed out that patterns of REM sleep and full wakefulness look remarkably similar.
While many people pride themselves on needing minimal sleep to function, Langarica explained why adequate sleep is truly necessary. “Sleeping four hours a night is not enough,” she said, as sleep deprivation can lead to skin lesions, severe pathology, increased food intake, weight loss, and even death. And in humans, sleep disorders can be linked to depression, diabetes, and autoimmune and reproductive complications. “There are a lot of experiments indicating that you become worse at learning when you chronically sleep less than you have to,” she said. “I’m the smartest I’ll ever be because I sleep eight hours a day, but maybe you guys could be smarter if you slept more.”
Langarica introduced the energy allocation theory, which explains the necessity of sleep. The body functions like a machine, she said. We use the machine when we’re awake, and maintain it when we’re asleep; a lot of maintenance happens during REM sleep. She then transitioned into thermoregulation and its surprising impact on sleep patterns. “Thermoregulation is the process that allows the human body to maintain its core internal temperature,” Langarica said. People maintain the internal temperature at which they’re most comfortable, and thermoregulation mechanisms return the body to homeostasis. Humans burn a lot of calories trying to sustain that temperature. “It’s the one thing we do as humans that spends the most amount of energy,” she said.
Interestingly, thermoregulation occurs during non-REM sleep, but not during REM cycles. For this reason, the temperature of one’s environment directly affects sleep quality; REM sleep is necessary for bodily maintenance, so good-quality REM is clearly important. “If the outside temperature that you’re sleeping in is not ideal, your quality of sleep will not be good,” Langarica said.
Langarica's sleep-wake model is driven by the circadian and homeostatic processes. Image credit: Alicia Prieto Langarica, AN17 presentation.
Next, she presented a sleep-wake model, driven by the circadian and homeostatic processes. The circadian pacemaker is the approximately-24-hour clock regulated by neurons in the suprachiasmatic nucleus. Throughout the day, the body’s temperature normally varies up and down by about half a degree. People tend to wake shortly after their body temperature begins increasing. Meanwhile, homeostasis simultaneously pushes the body towards a stable equilibrium. “The longer you stay awake, the more you want to go to sleep,” Langarica said. The remaining parts of the model represent neural components in the brain, while REM-off and REM-on signify which neurons are active during REM and non-REM sleep. Langarica uses Morris-Lecar equations—common in neural models—to simulate neural activity.
Different sections of the brain are active when one is awake versus asleep. For example, the preoptic anterior hypothalamus (POAH) is active during sleep. Langarica added temperature to the POAH in her model because it’s a REM-active area that is temperature-sensitive. Her subsequent equations included variables for metabolic heat production and dependence on ambient temperature.
When humans are awake, the body can thermoregulate at a range of 10 to 40 degrees Celsius (50 to 104 degrees Fahrenheit). But during REM sleep it adapts to the room temperature. Thus, Langarica added a waking mechanism that her model that accounts for temperature variations. Surprisingly, she found that 29 degrees Celsius (about 85 degrees Fahrenheit) is the ideal sleep temperature. “That’s the thing with biology.” Langarica said. “Everybody’s bodies are so individualized and different, and people are comfortable at different temperatures.”
In summation, Langarica introduced a sleep/wake model that included ambient temperature as a model input and body temperature as an output. Her model produces awakenings from REM sleep to thermoregulate via a temperature-dependent REM homeostat and an awakening drive to the POAH. “We see the same trends that we see in experiments on our model, which is what we wanted,” she said. She also determined that variations in sleep quality account for changes in sleep characteristics, such as REM latency and the number and length of REM cycles. Ultimately, her model supports experimental evidence of a strong relationship between thermoregulation and sleep dynamics.
However, much remains to be done. Langarica plans to further examine the dynamics of wakenings, analyze timescales, measure periodicity, and add stochasticity to her system. In terms of biology, she hopes to investigate the relationship between sleep deprivation and shift workers, understand the interactions between sleep history and sleep properties, and study the results of sleeping at so-called “good” versus “bad” temperatures for an extended period of time.
Click here for more coverage of the 2017 SIAM Annual Meeting.
||Lina Sorg is the associate editor of SIAM News.