A new research article published by the Science Advances peer-reviewed journal reveals that a person wakes up on average every five minutes. These disruptions in sleep are brief and last only seconds, so one usually does not remember waking up. The reason for this occurrence stems from a being’s biological way to protect and alert themselves of danger.
In the study, researchers revealed that small stints of electric activity in specific neurons, called the wake-promoting neurons, cause a person to jerk awake for a few moments before they fall back asleep.
While there is always some neuronal activity happening even when one is in deep slumber, researchers hypothesized that the specific firing of wake-promoting neurons is what causes the random sleep arousals. This occurs so beings can hear approaching predators even if they are asleep, because deep slumber hinders detection of foreign movement. Additionally, researchers proposed that since electronic activity changes according to temperature, the random sleep arousals decrease when temperatures surrounding a sleeping creature are higher.
To test all of these hypotheses, the researchers used zebrafish larvae. Zebrafish have comparable sleep cycles to humans but, unlike humans, they are ectotherms. This means they cannot regulate their own thermal systems, making it simpler to test whether the outside temperatures affect the regularity of their sleep patterns. The fish were tested in four different water temperatures — 25 degrees, 28 degrees, 31 degrees and 34 degrees Celsius. Thirty-one degrees Celsius is considered to be the normal water temperature for zebrafish to inhabit. For each temperature, 48 fish larvae were placed in individual containers filled with 1 milliliter of water for every larva. For 48 hours the larvae were placed on a schedule of 14 hours of light and 10 hours of darkness. The brain activity of the larvae was measured in one-minute intervals by an advanced tracking system that actively categorized the zebrafish as either being awake or asleep.
The study’s results showed that zebrafish woke up more frequently and stayed awake for longer periods of time when placed in colder water temperatures. However, the activity of the wake-promoting neurons in relation to the fish waking could not be determined, as there is no efficient way to measure the brain activity in animals that are asleep. Additionally, ectotherms are not easily comparable to humans, who are able to regulate their body temperatures in order to maintain a homeostatic or regulated core temperature.
Despite the differences between humans and fish, the results of the study allowed scientists to hypothesize a greater question; could the spontaneous arousal from wake-promoting neurons be the solution to sudden infant death syndrome? Sudden infant death syndrome is notorious for causing infants under a year of age to die without explanation, possibly due to breathing problems or suffocation. This could be due to babies having low neuronal activity, and thus being less likely to wake themselves up in the middle of night even if they cannot breathe.
“We came up again with a theory that the babies with SIDS have low neuronal noise and therefore they have lower arousals,” said physicist Hila Dvir, one of the researchers of the study from Bar-Ilan University in Ramat Gan, Israel. “Because they have low arousals, they are less protected from any hypoxic event — a shortage of oxygen.”
However, a correlation between core body temperature, sleep arousal and SIDS is not confirmed, and scientists cannot confirm what causes or prevents babies from dying of SIDS.
The temperature regulation of zebrafish and infant babies are similar in that they cannot control their own temperatures well.
“Because very young infants are more ectothermic than endothermic, their arousability could scale similarly to fish for different ambient temperatures,” explained Ronny Bartsch, another researcher who was part of the study and senior lecturer in the department of physics at Bar-Ilan.
Therefore, the results of the experiment could be used and expanded upon to compare sleep activity in babies and zebrafish, which could possibly lead to a real theory about why some babies die from SIDS and how babies can sleep better overall.