What is the origin of these arousals? Scientists from Bar-Ilan University in Israel and from Boston University, have discovered that brief arousals are probably triggered by the intrinsic electrical noise from wake-promoting neurons in the brain. Their research, published in the journal Science Advances, reveals a previously unrecognized neural mechanism that links sleep arousals with body temperature regulation, which may help solve the mystery of Sudden Infant Death Syndrome (SIDS) — a tragedy with no known cause, but many contributing factors.
During sleep, wake-promoting neurons, specialized brain cells that help us awaken, are suppressed by sleep-promoting neurons, specialized brain cells that help us sleep. Nevertheless, Hila Dvir, lead co-author of the study from Bar-Ilan’s department of physics, hypothesized that wake-promoting neurons still maintain a low level of activity — kind of like ‘white noise’ in the background. For individual neurons, this noise is very low. However, in aggregate, the noise can occasionally form a ‘loud’ enough signal that overcomes the dampening effect of sleep-promoting neurons and causes brief arousal.
Neuronal noise is very much affected by body temperature; if body temperature is high, neuronal noise is low, and vice versa. Dvir’s team tested whether changes in body temperature can affect neuronal noise to the point that waking and sleeping signals are disrupted, using zebrafish. The researchers analyzed periods when the zebrafish were predominantly sleeping and determined sleep duration and number of arousals in varying water temperatures. They found that, indeed, an increase in water temperature led to fewer and shorter arousals. (On the plus side, this made research easier; zebrafish’s body temperature can be easily manipulated by water temperature; on the downside, it means the findings are not directly translatable to humans, though the findings will likely guide further research that will be.)
The experimental results upheld Dvir’s original hypothesis, which had been modeled using a computer program before physical testing.
“Because of this excellent agreement between model predictions and the experiment, we believe that sleep arousals can be attributed to the neuronal noise of wake-promoting neurons,” says co-lead author Ronny Bartsch, also of Bar-Ilan’s department of physics.
The findings of the study present a possible new link between temperature, sleep arousals and Sudden Infant Death Syndrome (SIDS), the sudden, unexplained death during sleep of children under 1 year of age. Elevated room temperature, extensive crib bedding and prone sleeping position are known SIDS risk factors — and are all factors that contribute to higher body temperature. So far, the mechanism of why higher body temperature increases the risk of SIDS is unknown but neuronal noise and brief arousals could be a key. Since thermoregulation in young infants is not yet fully developed, their body temperature is highly affected by the environment/room temperature (similar to fish).
“We think that SIDS can occur when, as a result of higher temperature, neuronal noise levels and the associated probability for arousals are low,” says Dvir. “In contrast, when the temperature is lower, an infant has higher neuronal noise level that yield more arousals during which the infant can change his position to help himself breath more freely or move a blanket that may be covering his face.”
Time and much more research will tell. But it’s encouraging to have one more avenue of study in the effort to solve the problem of SIDS.