When we fall asleep at night and wake up in the morning, our brain switches between opposite modes of consciousness -- but scientists know surprisingly little about how these key transitions work at a neural level.
Now, though, we may have some answers. Research published last week in the journal Nature Communications sheds light on the neurological mechanisms that allow these processes to occur successfully. It also offers a clue as to why we sometimes can't fall asleep -- or stay awake -- when we want to.
Researchers from the University of Maryland School of Medicine are investigating, for the first time, the workings of a pathway that seems to play a key role in regulating the "switch" between sleep and wakefulness.
"If you want to understand how something as complex as sleep is produced by the brain, you have to understand the molecules within the brain that create specific patterns of neuronal activity that put the brain into particular states, such as sleep or wake," Dr. Andrea Meredith, a neuroscientist at the university and one of the study's authors, told The Huffington Post in an email. "Our study identified a set of central molecules (or ion pathways) in the brain's clock circuit that perform this encoding step."
These molecules control the sleep-wake cycle by regulating the pattern of electrical activity in the suprachiasmatic nucleus, or SCN. The SCN is a brain region within the hypothalamus -- a part of the brain responsible for hormone production, among other functions -- and it acts as an internal clock, governing the body's circadian rhythms. The SCN determines when we fall asleep, how long we sleep and when we wake up.
Ion channels are proteins that conduct electrical currents in different parts of the brain. The researchers already knew from previous work that certain ion channels, known as "BK potassium channels," were important for regulating electrical activity in the SCN. However, they believed that in human beings, the most important activity in these ion channels took place at night.
But the new findings reveal this not to be the case. In fact, they suggest that for humans, it's what these channels do during the day that's more important.
"The major surprise revealed by this study was that there is a specific biophysical switch mechanism, called 'inactivation,' that prevents the daytime BK channels from influencing neuronal activity in the SCN," Meredith said. "Without daytime inactivation of BK channels, the SCN doesn't encode the circadian time signal properly, and mice don't sleep during the day like they are supposed to."
In an experiment on mice, Meredith and her colleagues were surprised to find they could change the pattern of neuronal activity associated with daytime into a pattern associated with nighttime -- and that it was as simple as flipping this molecular switch.
Mice sleep during the day, when their BK channels are typically inhibited -- the result of this inhibition is high levels of neuronal activity that leads to sleep. When a mouse's BK channels become active at night, its neuronal activity is lowered, triggering wakefulness. Humans, of course, sleep at night and are awake during the day, but otherwise their association between neuronal activity and the sleep-wake cycle is similar.
Having a better understanding of this neuronal pathway might help us find new ways of treating certain sleep disorders like jet lag and insomnia, which involve dysfunction of the circadian clock.
"These channels could be specifically manipulated to correct jet lag, the clock de-synchrony that results when you travel across time zones," Meredith said. "It has also been shown that the circadian clock can influence mood disorders, and may underlie seasonal affective disorder, a type of depression linked to fewer hours of daylight in winter."
Sleep was once something of a mystery to scientists, but more and more research is shedding light on why we need sleep and what happens in the brain while we're at rest.
Another study, published Monday in the journal Nature Neuroscience, shows that memories formed in one part of the brain are replayed and transferred to a different area of the brain during sleep. The findings could tell us more about the brain mechanisms involved in Alzheimer's disease.
"This is the first time we've seen coordinated replay between two areas of the brain known to be important for memory, suggesting a filing of memories from one area to another," study author Dr. Caswell Barry, a biologist at University College London, said in a statement. "The hippocampus constantly absorbs information but it seems it can't store everything so [it] replays the important memories for long term storage and transfers them to the entorhinal cortex, and possibly on to other areas of the brain, for safe-keeping and easy access."