You go to sleep at night, you wake up in the morning—the definition of sleep doesn’t seem so complicated. But start asking questions and things start getting thorny: Are dolphins that never stop swimming sleeping? Are migrating birds that “shut down” half their brains sleeping? Is someone under general anesthesia sleeping? And what about babies in the womb?
Unborn human babies in the womb are pretty difficult to monitor 24/7, so the researchers interested in that last question got ahold of unhatched chicken eggs. In a new Current Biology paper, they report that chicks show higher-brain activity patterns similar to sleep, and the cries of a hen could “wake up” the chick even when other loud but not chicken-salient sounds could not. These higher-brain activity patterns only appear in the last stage of incubation, presumably after their brains become well developed.
To monitor brain activity in the chicks, the scientists carefully made a small hole in the top of the egg and injected radioactive sugars onto the egg’s inner membrane. The developing embryo absorbed these sugars, which the team could then track with a PET scan. Active neurons need energy, which they get from sugar, so mapping the radioactivity in the brain shows what parts of the brain are active.
When their higher brain areas were most active, the chicks themselves were actually the least active. Our brains, too, are very active during sleep while our bodies are not. Based on the chicks’ eye movements, the scientists also identified what resembled REM (rapid eye movement) and non-REM sleep stages. Muscles are paralyzed during REM—that’s the explanation for sleep paralysis—and the chicks were completely inactive during REM sleep-like stages.
If chicks can be asleep, it makes sense that they can be woken up. To test this hypothesis, the scientists disrupted the chicks with loud sounds, some of them chicken calls and others just random noises. Only the chicken calls activated their higher brain areas. A sleeping human will also be roused by hearing his or her name, but random sounds like a door creak are less likely to rouse even when it’s just as loud.
The parallels between “sleep” in unhatched chicks and sleep as we understand it in humans mean this could be a model for fetuses. The researchers are currently investigating whether being “woken up” before hatching is harmful to the baby chick.
[via New Scientist]
Hatching chick image via Shutterstock / Anneka